Aevey & Brownie, if you go through enough of the old threads you will find excerpts from American and Canadian statutes. You can also use Google to locate the full legislation if you desire. It is a bit of reading…
The really, really condensed version is that copyright exists to protect your original work from someone taking it as theirs and making money with it. It also protects your work from being used to create a derivative piece and making money from that, so you don’t lose copyright because it was edited. However, critique , education and parody are all part of Fair Use, which is an exclusion to copyright, and the copyright holder’s permission is not required.
Aevey, Imgur are covering themselves by having a TOS section, however, it sounds like they only act when pushed and in the case of Fair Use, the uploader is covered by “or otherwise have sufficient intellectual property rights to upload”, since no permission is required.
If you are in the USA and copyright is a big deal to you, you should spend a few dollars and consult a lawyer, or if your school has a law school, consult several of the better senior students to get their opinions. As a practical matter, if you go to court, the person or entity you are suing may counter file that your case has no merit and request costs, which would mean that if they win, you could be on the hook for their travel, time and lawyers fees. If you win, you may or may not get costs and the award is usually based on the money you proved you lost, or the other party was able to make, due to the infringement. It might be worthwhile going to court if your photo was used in an ad by a major corporation, it would certainly be worth using court as a bargaining chip in that case. If you have photos that are that good, you should be registering them with the Copyright Office. Going to court is expensive so you want to make sure you have all your ducks lined up before hand.