an item in the inventory screen that takes up a 2x3 space is 32x48 pixels, gif, bmp, png all acceptable, anything but jpeg. we need raw pixels, and jpeg compression sucks.
it needs to use a pre-defined palette of 256 colors, i'll pm you an existing graphic so you can get an idea.
the problem we face is that this will be an external graphic to the files we are using, so regardless, it will probably be a bitch to implement. until then, might i suggest we swap it from "plasma pistol" (32) to "dead human" (45) or "mind probe" (29), or as Daiky suggested, use tank turret graphics (40, 42, 43 and 54)