Here it is i think, ignore the forecolor/etc i only added that just so you make sure you place edit the right thing.
//BLOB
itemDef
{
name "blob"
rect 311 230 18 90
visible 1
forecolor 0.0 1.0 0.0 0.5
background "ui/assets/alien/tremublob.tga"
ownerdraw CG_PLAYER_POISON_BARBS
}
EDIT: rect is what you want.