Photoshop
Being the standard and therefore the application of choice, Photoshop is used for adding any effects to my game renders from 3D Max.
Photoshop would be used to add any effects and can then be used to resave as a different image format. Features I have used include:
Bevel or emboss
On many occasions I have used Photoshop to create holes for something like a vent texture using a bevel or emboss layer style.
Vector masks
These I commonly use to produce resizable shapes.
Button actions
I have my actions pallet set to button mode and have recorded actions for repetitive tasks like flip or duplicate rather than
keep going into the menu at the top.
Smart objects
The new edition to Photoshop, smart objects, has proven very useful for adding bitmaps to layers that can be re-sized without loosing
quality due to the original quality being stored in the Photoshop file.