I've always used photoshop, however I generally just use it for subtle colour editing, such as adding more contrast. I will say that I'm not much a fan of moc photographs covered in digital effects, because when I'm looking at your pictures I'm definitely more interested in seeing the quality of your moc, rather than to which degree you can paste fancy effects. Also, I've seen a good few examples of people attempting digital effects and failing, making the picture less appealing than it was before. That said, I don't mean to be all "don't add effects to your moc pictures ever" because really, it can be pretty fun. But it's usually just not relevant in the presentation of a moc.