Filesize itself wouldn't be a big concern at the moment in terms of our site's bandwidth - since our host claims to offer unlimited bandwidth and whatnot. However, it costs a load of time to download, especially for mobile devices (although we should get to work on a good mobile skin after we redo the site skin thingy) and slower connections.
Try various .jpg compressions, dunno if .png can be made to do a small file, and also try .gif for a change - it's quite good for graphics (i.e. line drawn stuff, not photos and stuff).
Also, is it just me or does the banner look on the tall side? I guess you could also reduce the size of the overlay graphics / site title and with that the total height of the image.
The rest, looks great and just,
.
Still not sure about the floating corpse though, it kinda looks off or something. Could you try some variations on visibility? I'm assuming you have it as a separate layer - try a few shots with gradually reduced opacity on that layer, see how that looks.
As for the rest of the forums appearance, I think we'd need some more visual aspects that add the glowing accents like in the banner. A logical addition would be category headers (you know, the bits above the forum listings), and maybe a subtle streamy faded swirly thingy behind the section listings - possibly one that crosses over the various sections. Not too obvious though - the site has to remain readable and whatnot.
There's a lot of possibilities, and if I had more time / skillz, I'd do some mockups myself.