First two are bland, and need to be better cut.
Whiteness shows around the edges, that can be fixed with a border, and a in shadow. The color scheme on the first two shouldn't look like gradients with two colors, and then a blend towards the middle.
The second one looks the same as the first, except it looks like an airbrush was used with white. You should learn to use renders, and different styles of rendering with lens, and warps.
The third would of been good if Goku was ontop of the black instead of behind it and then its opacity of that layer lowered for a 'see-through' look.
But, I'm just rambling from experience.
For a first try, their pretty good, but if you went to any graphic design forum, you'd be bashed to hell and back.
Some places for tips would be http://www.thedesignworld.com/
The Photoshop area has some nice things.