So on any fourm I try to use
this image as my avatar, I get an error saying the URL is invalid. I uploaded, copied the link, and pasted the link to/from Dropbox I the exact same way I have in the past with other avatars, but this one doesn't work for some reason. Any ideas as to why?
Because that's not an image; It's a web page.
You want to use a direct URL:
http://photos-2.dropbox.com/si/xl/z0sxQXgOO8ZK_7F1l_YGXwLMKCvwehGYQRM6eKh-zIU/61858450/1337619600/8306f0b/Photo%20Apr%2003,%2011%2054%2017%20PM.jpg
I set that as my avatar, but it's showing up broken despite working when I C&P it in the address bar of my browser. Is it broken for you, too?
It was working at the time but it seems to have expired. Try using a site such as imgur.com which gives you a direct link.