If you later need to edit the file, resize, resample, save in additional formats, etc., you and your graphics program will have fewer problems working with the file in its native format.
When creating images to be used on the Web in GIF or JPG format, editing and resizing is best done on the native file formats. GIF images don't resize well so if you need to resize, resize the original then export as a new GIF. Because JPG uses a lossy compression, data is lost each time you open, edit, and re-save the file. Edit the original file in its native format then export as a new JPG.
