Seriously. PNGs are at an incredibly higher quality of resolution. There's no contest... Save your "art" as .png to save our eyes. Thank you.
USEFULS.NET Blog Archive JPG vs PNG .jpg gives lower file size with lower quality, .png gives larger file size with higher quality. With images as small as sigs there is little noticeable difference, but why not go for the higher quality?
Well my mate says the same but i compared two of his artworks together and it seemed the same .. My sig is .jpg ..
Also, .png is a working file for when you're using Fireworks. (Not sure about other programs). That's why my sig's background is a bit crappy compared to the png. (Because you can't animate png's.)
I don't use JPGs often, but for my SOTW I had to because Photoshop wouldn't export it to PNG for some reason. Nontheless, PNG is superior in all ways.
I learned from my mistakes and now make my files .png. And to change em, when you are saving, do you know that little text box to name it? Well, where it says .jpeg, .jpg, etc. just delete it and type .png.