A couple of things I have noticed:
Bugs,
More than one image can be uploaded. The second image is shown, but it takes the same width as the previous image. If the second image is larger than the first one uploaded, it will be stretched and pixelated as a result.
Still some german phrases left that are not translated (you are not registered, for one).
Feature requests,
Either limit upload in a gallery to one attachment for the first post, or allow more images but with correct thumbnail sizes calculated per image.
Featured images. Admin can appoint specific images to be displayed on the forum home.
Thanks for this great hack btw. Well done