Just saying "draw a black box" is not sufficient, you need to know the implementation details. If the software saves in a layer-based format, that's no good. If

Just saying "draw a black box" is not sufficient, you need to know the implementation details. If the software saves in a layer-based format, that's no good. If there is alpha channel then it's no good. If there is pre-existing compression artifacts that can leak information. You basically need to know that it does the dumbest thing possible when editing the image. I guess mspaint is probably the best option.