What I will explain may seem technical, but it is to maintain accuracy and not confuse slang terms. I'll do a set-by-step discussion format to make it simple.
First, a .GIF and .JPG are image formats that conform to a given standard. There are several approved variations of each format, too.
Second, sofware ASSOCIATES itself with the extension (e.g., .JPG or .GIF or .BMP, etc.). Usually, the most recently installed software will be the one that has set the some (or all) file associations for its own use.
Third, most software will provide a pull-down submenu that will present a widnow that has an option to set all (or some) of the image extensions to be associated with that software.
Fourth, most modern image viewing/editing software will let you load one image format (like .GIF), then save in another format (like .JPG or .TIF). Each software may handle the mechanics a bit differently.
Personally, I use Adobe Photoshop 6 for "heavy" editing, and ACDSEE 3.1 for viewing, changing formats, and basic editing. I do not allow MS image software to get anywhere near my photos, images, etc.