if it doesn't work attach the picture and i'll save it to my computer and upload it then give you the IMG. And to save it as a different type, just open the pic, go to 'Save As' and save it as you normally would, although change the file type. Do what sear said, make it a jpeg file, although i save everything as PNGs.
Ok, create an account at www.photobucket.com , then click on the "upload" button and chose that file. Then Photobucket should give you a link to it (underneath it, there will be 3 in fact, highlight the one that says [img] copy and paste it). Then goto your signiature and paste that. If that doesn't work or you don't understand all of it send me a PM and I'll try and help.