I found this thread at Microsoft Answers, in which someone was trying to get help on making PhotoShop their default picture editor. Note: As the op was trying to explain to people, they wanted to change the default Editor, NOT the default Opener.

A few tedious misunderstandings later, there was this at the end of the discussion, which was the answer:

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For the record, the registry edit is-
HKEY_CLASSES_ROOT\SystemFileAssociations\image\shell\edit\command

Change the default of mspaint.exe to your image editor of choice. In my case, Paint.NET.
Example:
OLD-"%systemroot%\system32\mspaint.exe" "%1"
NEW-"%ProgramFiles%\Paint.NET\PaintDotNet.exe" "%1"

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^^ For me, the new string was

"C:\Portables\IrfanViewPortable\App\IrfanView\i_view32.exe" "%1"

Hope this helps, somewhere.