I looked a little further into this. Currently you're using a simple PNG with the text on the background (
http://content.specialattack.net/motd.html). So it's
not scaling correctly as you just wrote in the howto.
I'm running TF2 on a 1600x1200 resolution (minority... I know) and so the fixed image gets really small and hard to read. Wouldn't a better approach be to use regular HTML (and em'ed font-size) instead? The only drawback would be that you cannot select the font you wanna use. Or won't this work at all?
Otherwise... over 70% of the Steam users run 1024 or 1280... so the current solution should be fine.