There are several programs out there that allow you to decrease the color depth of your images without losing much, if any quality.
One in particular, IrfanView, is a handy, low resources program that does just that. I'd recommend giving it a Google search, downloading it, and using it to make your imports valid. Once you have the program, it's as easy as clicking Image-->Decrease Color Depth-->256 colors.
If you have a program such as Photoshop, you can convert the image by going to Image-->Mode-->Indexed Color and selecting a 256 color format.
Hope that helps, then again, this question has been asked at least 2.6 billion times >.>