Ive heard of gimp, aparently its very much like photoshop, but i havent tried it myself. If gimp is anything like photoshop, it'll have layers, and somewhere in one of the layers menu will be different blending modes, it changes how the colours on the layer react to whats underneath that layer. Duplicate the layer with the trooper in it and change the blending mode to something, like overlay. Or you could change the layers opacity to make it more transparent. I dunno if gimp would have it, but look for something called layer mask near the layers, if its there, it allows you to create sort of a layer which governs where the image can be displayed where you use white it shows the picture, the black is completely transparent, in-between effects the opacity. Its nice to sometimes use a gradient on a layer mask....
Hopefully that was of some help to you
or gimps features arent like photoshop. The sig itself isnt that bad, just the image isnt blended well, also i think you've over used the tech brushes, theres no need for the stuff in the middle and at the bottom, but maybe something more to the right hand side. I'd also get rid of the white text behind the trooper, he's the focal point there the contrast of the harsh white is too distracting. If it just had the dark background there it'd be much nicer, especially if u blend the trooper in more. I like the way u got the trooper pointing towards those circles =P nice, i like the font too, just i'd lower it a bit, maybe put it to the left a little, not sure. Anyhows good luck, i hope ive helped D: