It's possible that the file has been given the wrong extension. Some people for example rename "image.gif" to "image.jpg" thinking that this is all that's necessary, to convert a file.
Fortunately, it may be possible to determine what kind of an image this is. If you open the image in notepad or some other text editor, the correct designation of the file can be seen.
GIFs tend to begin like this: "GIF89"
JPEGs will look something like this: "ÿØÿà JFIF"
PNGs start like this: "‰PNG"