I use Fraps.
Then just wack it into a jpeg instead of a bitmap.
You can chose to write as jpg in the first place ya know?
Use Fraps, You get the Best quality screenshots with them.
prt scr button, then use paint .net to paste it into and change the size and jpeg it.
A screen capture is a screen capture, regardless of the application that takes it. The quality will only ever be that of what is rendered by your PC, and will be the same no matter what method you choose to extract it.
For me, either the ingame screenshot button, or PRT SCRN.