(This is for windows, as far as I know) Go to your photo editor program, and open the sig. Go to File, and Save as. Under the name of the picture, there should be a pull down bar. Change it to JPEG (.jpg) and then resave the file.
If you seriously cannot do it, feel free to email me at Celestial1@virgindata.com. If it is a graphic signature, don't bother sending it (I have my precautions). I will change it to JPG and even resize it if you need it (Provide a height or width pixel length if you do). Of course, and this is a forewarning, if I don't notice it immediatly it could take a day or so.
~Celestial