I emailed a leopard egcko breeder of one of my geckos to find out what morph he is, and they asked for a JPEG. What is it, what does it do to the picture, and how do you use it?
JPEG is a kind of photo compression, a very common save/output style. If you have a GIF, BMP or PNG image, you should be able to use a free editing program like IrfanView to change it to a JPEG/JPG
if you use windows, it should have paint on it. open the photo in paint and "save as" whatever you name the picture and from the click down menu choose jpg.