a few suggestions, i assume your using photoshop, prolly 7.0 or CS right? so why dont you try using some blend modes on the different pics. also, to get the background on the pics to fade more to the black background, instead of just stopping, bust out your eraser and lower the opacity to about 15% or so, and keep going over the edges to soften them and blend them into the background. also, maybe loose the halo glow around the BMW symbol and text, its kinda cheesy
edit: dont for get, in a collage, the pics can overlap, which might make it a more visually interesting compsistion overall