Apologies in advance if I'm 'teaching my granny to suck eggs' (do you know what that means or is it a peculiarly Scottish expression?):
JPEG's not a good format for graphics with simple block colours and text. The colours come out blotchy at smaller file sizes and the text doesn't look crisp (you can see this in the screenshot in your top post).
For simple things like windows dialogue boxes, logos with block colour and your forum screenshot, GIF and PNG are much better. At 256 or (even better) 16 colours the file sizes come out low as well. Where JPEGs come into their own is for photographs and images with gradient colours. True colour PNGs also make beautiful photographs but the files are usually far too big for the web.
BTW, I'd rarely go below 16 colours even for black and white text because you still need some variation in the available colours to anti-alias (smooth) the edges.
Sorry if I haven't explained it very well or you knew it all already! Perhaps there's a German site somewhere that deals with all this.
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