How do you tell?
Well Z4go's image is clearly pixelled. You can see that it was drawn one pixel at a time. Same with Slotman's image. ShadowTurtle's image shows signs of being blurred, airbrushed and other things which paint doesn't have. It's not 100% conclusive, because you *could* pixel in Photoshop if you wanted to. But to my eye, every image except ShadowTurtle's ( and he admitted he used Paint .NET ) looks as though it *could* have been done in MSPaint.