Last I heard, TuxPaint worked fine on Windows 7 64-bit systems for some users but not for others, though none of the errors were personally observed. I'm CC'ing tuxpaint-users mailing list that should have more folks that are knowledgeable on this matter.
I hope this finds you well.
I know you are US based, but I cannot find a UK contact and the guy in France was not very helpful;
I just want to confirm if Tux Paint is compatible with Windows 7 64 bit?
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