Not all images are invalid. Perhaps some different reasons to make images valid or invalid from imgur.com.
Can you point out some of the image links from imgur that are showing well in the forum?
Because I just checked my post history and from the few that I have seen, I believe all images that are hosted on Imgur are having the image proxy error. I have been uploading mine on Imgur this entire time, and it hurts if they are going to delete everything.
Images hosted on other sites like imgbb are OK.