your best option is to check where your site is listed with bluecoat http://sitereview.bluecoat.com/sitereview.jsp
Thanks for that link! I've been wondering the same thing (some of my sites have to do with "Gaming" which a lot of school filters like to block). Interesting to find that LightSpeed ranks one of my sites as "R" rated. Bastards!
I could always test them for you if you wanted. Keyword filters for schools always block Gaming, Proxy, Music, and other random sites.
not sure how to use lightspeedsystems link (maybe the website had changed meanwhile) but bluecoat link works. For some reason it marks my site https://www.unblockedgamescentral.com/ as "suspicious" but not gaming, which is maye even better. I think it's been marked like that because it utilizes https protocol.
You could try a tracert to it. If it stops before it leaves your network, then it's being blocked. However, a possible workaround a block is using https, instead of the standard http. This only has a chance of working if the site supports it. In you're case, imgur.com does support it. Try this link: https://imgur.com