Posting to provide a revision to the registry key value and to also confirm an OS working (Windows 10 Version 1607, OS Build 14393.576 - AKA Anniversary Update)
They key listed above is valid, but assumes that a user of a 64-bit Windows is using 64-bit Paint.net and that the Program Files path is in the c:\. Though unlikely that someone will install a 32-bit version on a 64-bit OS, it is possible by extracting the installation package with a program like 7-Zip and installing with the PaintDotNet_x86.msi. Haven't tried it, but I'm a stickler for portable code and registry snippets like this :-D
I used the same key/value/etc., I just changed the beginning of the path to use an environment variable provided by Windows.
For reference, this is the default value in its raw hex format. You can put this in a .reg file of your own and merge it to restore the default:
Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\SystemFileAssociations\image\shell\edit\command]
@=hex(2):22,00,25,00,73,00,79,00,73,00,74,00,65,00,6d,00,72,00,6f,00,6f,00,74,\
00,25,00,5c,00,73,00,79,00,73,00,74,00,65,00,6d,00,33,00,32,00,5c,00,6d,00,\
73,00,70,00,61,00,69,00,6e,00,74,00,2e,00,65,00,78,00,65,00,22,00,20,00,22,\
00,25,00,31,00,22,00,00,00
or, if you want to just copy and paste to restore the default value/behavior, use the following:
"%systemroot%\system32\mspaint.exe" "%1"
When editing the existing registry key value located at [HKEY_CLASSES_ROOT\SystemFileAssociations\image\shell\edit\command], edit the value and paste the following:
"%ProgramFiles%\paint.net\PaintDotNet.exe" "%1"
If you want to merge a registry file with your registry to overwrite the existing value, use the following in your own .reg file:
Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\SystemFileAssociations\image\shell\edit\command]
@=hex(2):22,00,25,00,50,00,72,00,6f,00,67,00,72,00,61,00,6d,00,46,00,69,00,6c,\
00,65,00,73,00,25,00,5c,00,70,00,61,00,69,00,6e,00,74,00,2e,00,6e,00,65,00,\
74,00,5c,00,50,00,61,00,69,00,6e,00,74,00,44,00,6f,00,74,00,4e,00,65,00,74,\
00,2e,00,65,00,78,00,65,00,22,00,20,00,22,00,25,00,31,00,22,00,00,00
If you accidentally deleted the (Default) value in [HKEY_CLASSES_ROOT\SystemFileAssociations\image\shell\edit\command], create a new REG_EXPAND_SZ there, and give it a value of (same as the first example):
"%ProgramFiles%\paint.net\PaintDotNet.exe" "%1"
Want to go hog wild and just be different for the hell of it? (Or you like more specific context menus) Use the following key to merge into your registry. It's the same value above, but it puts it into your Right Click/Context menu on images as "Edit with Paint.net" and doesn't mess with the Edit item in the menu:
Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\SystemFileAssociations\image\shell\Edit with Paint.net\command]
@=hex(2):22,00,25,00,50,00,72,00,6f,00,67,00,72,00,61,00,6d,00,46,00,69,00,6c,\
00,65,00,73,00,25,00,5c,00,70,00,61,00,69,00,6e,00,74,00,2e,00,6e,00,65,00,\
74,00,5c,00,50,00,61,00,69,00,6e,00,74,00,44,00,6f,00,74,00,4e,00,65,00,74,\
00,2e,00,65,00,78,00,65,00,22,00,20,00,22,00,25,00,31,00,22,00,00,00