Creating imagery for the web is nice balance of quality over file size. You have a choice in file types of either a JPEG or GIF and what to use when totally depends upon your subject matter in the image.
For instance, if your image is mostly photographic like a sunset scene or a portrait of a person, we call this RASTER based and always use the JPEG file type because JPEG’s can handle thousands more colors than a GIF can. If your subject is illustration based like a company logo this is what we call VECTOR based and always use the GIF file type. If your image contains a mix of photographic and illustration subjects (Raster & Vector) JPEG is a safe bet. HINT: always save your JPEGs at 60% to 70% compression for optimal quality and file size.