Hello,
It has been solved over an e-mail conversation (hopefully). Here is solution for other users.
It turned out that it was issue just in version 5.0. Image attachment gets cached with the proportions of
Attachment image maximal side size value within
Forum Group webpart properties. You can try setting this property to say 1000, saving webpart properties and then reverting it back to your original value. After this "workaround" it worked fine.
At any rate, this isn't issue in version 5.5 of Kentico
CMS, therefore another solution is upgrading to this version.
Best regards
Ondrej Vasil