Gimp rocks !
I never feel like missing Photoshop because its been ages when I last used Photoshop.
Here is a little step by step tutorial for creating a 3D box in Gimp.
Go to File>>New (Of course !) and create a layer with white background, and open any image as first face of the box(cube). Make that image square sized if its not.
Open the other face of the cube and align it with one of the edge of the first image.
Use the Perspective tool and alter the latterly added image.
Add another image for the third face i.e. the top face and using
perspective tool align it two already placed images.
Try zooming into the image for perfection at edges.
Get some pop corn and enjoy watching your cube/3D Box ! :P