Oh! I just had an idea!
The images over on The Cover Project are actually 300dpi. You could probably just crop the part you want to use and blow it up to an 18x24 print and I suspect it would actually look pretty good.
I do apologise for resurrecting this. I was looking for poster art when I came upon the thread.
It's a few years, and maybe the OP isn't interested anymore, but I found this very hi-res Metal Slug art (5021 × 3544) so thought it might be useful. If not for the OP, the perhaps for someone else.