Get Photoshop cs6, because fuck GIMP(it does more harm than good to a beginner).
Get this: 'Edit Layers in ACR 1.1.7', ctrl+f to find it easier.
With this you can use the tool used for editing .RAW photo files, contrast, brightness, whitebalance, clarity etc etc. Just fool around with the sliders to understand what they do. You can do this with filters etc in PS, but this saves so god damn much time.
AND in acr you have the 'Adjustment Brush', looks like a brush, with which you can "paint" contrast/brightness/clarity etc. Use it to bring out a bit more clarity in eyes, dick, whatever.
Use the 'Clone' tool in ps to get rid of those pesky broken joints or whatever. You can also use the 'Liquify' tool to smooth out sharp knees or simply enlarge breasts/similar.
Add some additional depth of field with the 'Blur' tool.
I've been a hobby photographer for 3 years now or more, so I know how to compose and expose a scene. So this obviously helped me with setting up an interesting image in SFM. Making your scene look as good as possible before post-processing helps you a bunch. Depending on what you're aiming for of course.
This is a good example:
I wanted to have the feel of her being alone in a dark void, so only 1 lightsource and nothing visible but her and the chair.
I fixed her body up because of the limitations of sfm/gmod models and simply added some contrast and shadows, painted less 'Exposure' and more 'Shadows' with the adjustment brush in acr to vignette the image bringing more focus to her chest/face.
Remember to use the depth of field(aperture in the camera setttings) to make your model stand out from the rest of the scene, make her pop with an "advanced" light setup(1 behind, 1 front/above/or both, and 1 on side) and set a good camera angle. Don't be afraid to use a longer focal length(zooming in), wide angle only works sometimes.
I ended up writing so much, you better understand what I wrote :twisted:
PS: Don't overdo it, too contrasty, too colorful, etc. It should be subtle yet noticeable!