Maybe it's just the internal state of Photoshop. Try saving it first, of course that negates the reason for pating in the first place.
If it has a drag and drop feature, you could try that.
It could be but I find it very weird that it works when pasting onto MSPaint and every other program but not in C2.
In any case I don't think the Scirra team has much control over the image editor (I think I might have read that before somewhere on the forums) so I'm not sure this would be fixable anyway...
Edit: After a lot of testing I couldn't get it to work so I decided to use C2's "external image editor" option to open my images in graphicsgale, then copy/paste it there. It takes a bit more of time, but that's all I could do..