Hi Kevin,
Images of different sizes are obviously different. I don't remember about resolutions, but even if TestComplete can convert images to the same resolution, the result of conversion is unstable because it includes floating-point operations.
You can play with the comparison mask, color tolerance and other parameters described in the help system, but in general, images comparison is pretty unreliable and should be used only if you cannot get to the properties of compared objects in any other way.
Otherwise, it is much more reliable to use property or object comparison checkpoint instead of the image one.
