It looks like imgur is no longer blocking the image proxy. I didn't change anything on the forum's end, so imgur must've loosened up their limits. I guess it's not necessary to convert img images into links as long as imgur keeps working. But to prevent this from happening again, you'll get an error if you try to embed imgur images in posts from now on.

I'd like to create a page which recommends services for hosting forum images, and mention this in the error message you get when you try to use imgur. Other than TalkImg, which ones are good?