The forum and the new site are not connected in any way whatsoever. There may have been some change in Discourse itself, but I looked in the changelog and didn’t find anything.
When I click on an image, it does indeed open a blurry version of the image as the background, but it opens the correct resolution image in the foreground. If the comment has multiple images, it makes a little gallery where you can go through the images. It’s working just fine on my iPad and iPhone. I’ll test it on more devices. It needs to be reproduced with certainty and more information before a bug can be reported to Discourse.