That's because you probably didn't know that a .jpeg recompresses the pixels and resamples every time you open and re-save them. That's why it's garnered the term "Lossy" file. A .png is still a low file size raster image but is considered "Lossless" in terms of recompression.
Yes, a .jpeg image loses image quality over time. That's why I refuse to use them or give them to a client. We have a sign with a pixelated .JPEG word and the international "NO" sign over it... under it says, "Friends Don't Give Friends JPEGS"
Even Apple has adopted .png as their defacto raster image of choice when you take a screen shot. That, and there's actually the potential for licensing issues with .jpeg, since the word J.P.E.G. actually stands for Joint Photographic Experts Group. Same way .GIF wanted to pull some crap a few years back and charge everyone for using .gif as a standard web image. Since the .gif protocal is owned by Unysis, and they wanted to pull some patent licensing nonsense, developers and OS creators are steering clear.
Order of file importance in the Printing/Graphics World:
.EPS
.TIF
.PDF
.PNG
.BMP
.WMF
That's it. (Even the last two are marginal at best, but a lot of regular PC users can open and view them with standard Microsoft documents)