In subsequent testing I discovered that pasting from GIMP to PaintDotNet always fails. (whether or not a clip catching program is involved). Any attempt to paste to PaintDotNet gives this error:
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Paint.NET
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The image in the clipboard couldn't be recognized. Try re-copying it with the original application that was used to acquire it.
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OK
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Note same clipboard contents (from GIMP) can be successfully pasted to mspaint.exe, excel, and Irfan. If re-copied from one of them it can be pasted into PaintDotNet (so at least there is a workaround)
version of Gimp used is Windows 2.4.5
FWIW - a programmer (of another program ClipTrak which I'm beta testing) told me "GIMP uses a bunch of formats (largely in MIME format). To fix the problem above I used their 'image/png' format which gives good results (since the image supports transparency)"
Is this something PaintDotNet can/should take care of - on the principal that one should be as liberal as possible on import and conservative on output. (I know lots of people feel Internet Explorer is MUCH to liberal in what it attempts to accept but that's another story )
To summarize for my own use and followup I made this table of what currently works and what doesn't work:
