It looks like when a sprite/image doesn't occupy all the space assigned to it in memory, it's dumped with left over gibberish from previous draw calls, which causes the same images to be dumped hundreds of times and thus make replacement impossible.
This image alone was dumped a couple dozen times in the span of a few seconds, you can see how the bottom pixel changed color a bit, causing the emulator to dump it again.
It looks like when a sprite/image doesn't occupy all the space assigned to it in memory, it's dumped with left over gibberish from previous draw calls, which causes the same images to be dumped hundreds of times and thus make replacement impossible.