yes itc an be, your image may only have 1 color in it, but its palette doesnt, you likeley still have it in 32 bit color you need to knock it down to 8 bit, i forget how to do it off the top of my head enough to give you instructions, (and im too lazy to open photoshop and tell you) but as many people here ran into that problem before and ihad to help them with im sure someone can help you