Re: Megan fox
Focus on a more distinctive colour scheme - having hues from all over the spectrum don't work together well. Also, having fluorescent yellows and greens are usually a no-go as well.
Compositionally it is well-structured - although it does lack any realism and makes little sense. When placing an image, stock or render in a signature, try and place them in a believable environment - surround them with secondary imagery that builds on this and also compliments the main focal but doesn't draw any attention away from it.
As Verts said, the far left needs work - avoid leaving large empty areas, always have something visible.
Finally lighting. Firstly, create a single defined (or occasionally multiple) light source - this can come from an orb, or behind the render, or possibly just a simple glow from a source outside of the visible canvas - and then adjust the entire signature to fit with this, this means adjusting the location of the light and all other objects in your piece so that the shadows and highlights fall realistically.