If your image isn't proprietary, I'd recomend uploading a sample to an image
host like photobucket to show us.
Without looking, a very useful tool for me has been color>color to alpha. This
will require that you have the alpha channel enabled ( iirc -- posting from my
phone -- can be done through layers>transparency>add alpha channel.
If you play around with this feature a little bit you'll figure its strengths
and weaknesses out quickly.
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