Images should be optimised to reduce file size. Reduction in file size reduces download times and saves server file space.
Images to be used for the web can be reduced in resolution. Resolutions between 72 and 100 pixels per inch are suitable for most monitors. Higher resolution should be used if the image is intended for print.
After editing the image reduce the size to the same as the size it will appear on the web page. If the image is to be 200px wide and 100px high then upload an image that is 200x100. Reducing the file size after uploading increases the amount of time it takes for the image to load.
Image editing software provides the ability to compress images. For GIF this means reducing the amount of colours used. For JPG a compression setting of 50% is the minimum recommended without too much quality loss.
Interlacing means that when the image loads every second line will be filled in and then the remaining lines will be filled in. This is recommended for bigger images because most of the image is visible even before the image has fully loaded. Non-interlaced images load top to bottom without leaving a line.
Image editing should occur at the highest resolution, colour depth and image size as possible. This means editing from a large file and then optimising it after the edit. The image holds enough image data to make good editing decisions. When the editing is complete optimisation of size, colour depth and compression can occur.