There are a choice of three formats that can be used for graphics images for use on the internet. None of these formats is suitable to be used for work in progress so you are best off using a different format for the file while you are working on it and then convert to one of these internet formats once you have the image ready to publish on the internet.
Keeping file sizes to an absolute minimum is essential for files that are to be loaded to the internet.
This is the format to use for photographs (and other images having more than 256 different colours) that you want to load up to the internet. This format stores the maximum in image information into the smallest possible file and you even get to select the compression ratio yourself so that you can exchange a small (but acceptable) loss in picture quality for a dramatic reduction in file size. For internet use, a compression factor between 50 and 70% should be acceptable.
Indicative file sizes:
This format allows a maximum of 256 colours and so is only suitable for images where lots of colours are not required. You might ask therefore why you would use it instead of jpeg. Well there are three advantages that this file format has over jpeg. These are that it supports transparency and animation, and that in some circumstances it can produce much smaller file sizes.
Setting the background of the image to be transparent allows images to be placed into your page that are not just plain squares but can be whatever shape that you want. The colour that you select to be transparent will be transparent across the whole image so this format is also useful for line drawings etc. The images on this page are examples of gif files with transparent backgrounds.
It is also possible to store multiple images within the one gif file and set them up to display in sequence with varying length delays. This allows the creation of animated banner images without any special coding required as the animation is built into the actual image. Special software is required in order to create animated gifs. Animated images are slightly larger than the combined size of the images contained within them and so will take longer to download than normal images - you should try to avoid using them wherever possible.
Where your gif does not need to use 256 colours, the image can be reduced in size by reducing the number of colours in the colour palette built into the gif. One example of this is the single pixel transparent image that is used on these pages to help control spacing, by only supporting a single colour in the palette the file size is a mere 42 bytes. The file would be even smaller if the image was not defined as being transparent.
This format was introduced with the intention that it replace the gif format. As well as supporting more than 256 colours, this format not only allows you to define one colour as being transparent but (by using what it calls an alpha channel) it also allows you to define levels of transparency so that the colours in your image can appear with the background showing through as well. This can be particularly useful when you use style sheet positioning to control the location of the images on your page and are overlaying images over one another or over the page text.
The main disadvantage of this format is that support is restricted to the more recent browser versions as this format was only recently introduced as a valid internet format. For this reason, it should be avoided where you are looking to create pages that are supported by the widest possible range of browser versions.
If you are sending someone a graphics image that they will look at but not want to edit themselves then one of the formats mentioned above for use with the internet will best suit your purpose. If on the other hand the person that you send the file to is going to want to edit the file then you will need to select a file format that is supported by both your and their graphics programs and which does not lose data each time you save the file. Assuming that you are not both running the exact same version of the same graphics program on the same operating system, the native file format of your graphics program may not be suitable. The following file formats provide the greatest support across different graphics programs and/or operating systems.
This is the standard format used by Windows for storing graphics images. This format does not use any compression whatever so files in this format are HUGE compared to other formats. All versions of windows come with a program called paint which can be used to edit files of this type so even if you don't know what graphics program the other person might have, you can use this format to send graphics files to anyone running a version of windows and know that they will have a way to view and edit the image.
This format is not supported on all non-windows platforms. Additionally, the huge size of files stored in this format makes it difficult to find media that can be used to transfer the file between computers not on the same local area network.
This is the standard image format for the Apple Macintosh. While this format produces files that are smaller than BMPs, the format is not supported by all graphics programs on other platforms.
This is the format that was designed to be used for trading graphics files between graphics programs and operating systems. This is therefore the best format to use for exchanging files where the other person is using a different operating system or a different graphics program. All decent graphics programs support this format.
This format supports optional compression so that you have the opportunity to choose whether to make the file smaller and perhaps lose some cross platform compatibility. If you do find that your compressed file is incompatible with the other system try again without compressing the file.
The best file format to use to save your original copy of the graphic to allow yourself to work on it later is the native file format of whatever graphics program that you are using. Each decent graphics program has a native format that stores all of the information about your graphic so that if you save the image and then reopen it later you will find yourself in exactly the same position as if you had never closed it in the first place. For information on some of the graphic formats used by particular software see the list of graphic formats below.
Software specific formats are also ideally suited to exchanging files with someone who is running exactly the same version of the same graphics program on the same operating system as yourself but may not be compatible between different operating systems, different graphics programs, or even different versions of the same graphics program. Many of the best graphics programs do support reading and saving files in the native formats of other graphics programs although they may not support all of the features that the other graphics program does and so the graphic may not appear exactly as expected.
The following are some of the many graphics formats that exist. Many of the formats listed are supported by the more popular graphics programs. (Note that this is by no means a complete list of all of the different graphics formats that exist).
This article written by Stephen Chapman, Felgall Pty Ltd.