I am new with computer programs, however, I use Paint (very small, and simple).. Cropping I personally think does the biggest change in the picture.. When I crop my picture, it helps the viewer focus on what I want them to.. Not what their wandering eyes focus on..
Maybe cropping your pictures so their eye doesnt wander around the backround of the picture would be great.., also, try blurring the backround, maybe using a small aperture (try getting her head in a shot, with the backround NOT close nearby, so it blurrs it, but have it do something with what the picture's trying to "say" to the viewer)
-Jess