What resolution to use for your images depends very much on what you are intending to use the images for. It also depends to some extent on what the image is.
A general rule of thumb is that images which are only going to be displayed on a computer screen should always be set to 72 ppi (pixels per inch) resolution because this is approximately the resolution that computer screens have when the screen resolution is set appropriately to best match the physical screen size.
For everyday purposes, images which are to be printed should use 150ppi which is a good compromise between image quality and file size.
For high quality printing you probably want to go to 300ppi. For most images this is the highest resolution that you probably should use as using a higher resolution than this makes for much larger files for little improvement in the quality of the image.
Line art is a special case because of the clear boundaries between the black and white sections of the image. In this instance you may want to use 600ppi in order to get the clearest image. As line art only uses two colours, a line art file will take up much less space than a greyscale or colour image.
It is very simple to calculate how big a given image file will be prior to compression being applied. To do this all you need to do is to multiply the width of the image (in pixels) by its height (also in pixels) and then multiply it by 3 for true colour, 2 for high colour, 1 for 256 colour or greyscale or divide it by 8 for lineart. The resultant number is the size of the uncompressed image in bytes. Depending on the file type that you use to store the image there will be additional bytes added to this to store information about the image and if a compressed format is used the image size will be reduced by removing redundant information resulting in a reduced file size. Different file formats will produce different file sizes when storing the same image.
To convert between actual image size (in inches or mm) and the size of the image in pixels you need to multiply by the resolution of the image in pixels per inch or pixels per mm. To convert from size in pixels to size in inches or mm you need to divide by the resolution.
You can resample an image to change its resolution. This works better when you start with a higher resolution and are going to a lower one. You need to remember when increasing the resolution that the computer cannot add the extra information to the image to makes the higher resolution image clearer than the lower resolution image was.
This article written by Stephen Chapman, Felgall Pty Ltd.