there would be two ways, one by rescanning them in as a jpg, or two, by opening them in a photo editor, such photoshop (the top end), or there are a bunch of free ones out there too. then from there, export the pictures to jpg's.
I'll have a look see for some of the other editors available tomorrow and let you know what I find. If you have Microsoft office, it would come with a half descent editor, it doesn't do as much as some of the big ones do, but it would let you "export" or "save as" to other formats.