While a simple ADMX and ADML for paint.net are relatively easy to create, it's likely that they wouldn't actually achieve your desired result, due mostly to how GP works.
When an application isn't directly aware of group policy, such as paint.net, and is just reading registry values, your ADMX might be able to set those values to what you want them to be, but the user will be able to override them from within the application itself or by using regedit.
And, since policies don't re-apply (without other supporting configuration), your setting would only be good as an initial preference. At that point, you may as well simply set the appropriate options at install time or modify the MSI tables using orca or something like that to set those values to whatever you want on install.
