I have this image that Im laying atop another image. They are both gifs.
Im using the eraser tool to erase at 47% the image to make it
transparent but, above 50% its 100% and below 50% its 0% transparent.
Gif only allows full transparency or full opacity.
If you want to have any other values for transparency than
0% or 100%, you need to use a different file format (e.g. png).
Also, as already mentioned by others, make sure the image is
in RGB mode.
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