When you design graphics for the web, your main concern is file size, which translates into loading time. You always want to minimize loading time, but still having enough quality bytes to see the image clear.
One way of optimizing your file size is using Adobe Photoshop, which gives you the option to “save for the web”. This will optimize your image and reduce file size as much as possible. You can save your images in different formats like gif, jpg, or png, which is good because jpg doesn’t support transparency (only gif and png-24 do).
If you don’t have Photoshop, you should try to make your image as small as possible, preferably less than 56 Kb in size. Many people in rural areas still don’t have high-speed internet, so for them it still takes about 1 second to load 56-128 Kb of data (as a rule-of-thumb).
NEVER post images larger than 1 Mb on any website, as this will take forever to load!