So this may not be the soundest advice I've ever received, but a very wise man once told me regarding copyright issues, it's better to apologize than to ask permission. Really, you can publish whatever you want online, but you have to take it down when someone asks, or else you'll get sued, he said. This is, at its core, true: as it says on the Measuring Fair Use site, the only way to know for sure if it's fair use is to get taken to court.
While I don't follow that advice, and have taken similar precautions to our friend Ariel in the text book, I've also made decisions that have a bit of gray area. This is somewhat anecdotal, but once I used a plain text file that was self-published by an English literature professor in Japan. His site hadn't been updated since 1998, listed no site administrator, and his name was too common to effectively google. So I just took it and cited it as best I could.
What I did was fair use, as he hadn't made any creative changes to a 19th century text, just republished it. However, the changes I made to the text did necessitate me giving it a Creative Commons license. I went with Attribution Non-commercial. But I didn't do Sharealike. There are a lot of arguments for why you should or shouldn't do Sharealike, but the big one has to do with money. I don't care about money because my projects were academic. But for artists, Sharealike is much more important.
I should clarify, another word for Sharealike is "copyleft". Arguments about copyleft vary. Some people feel really strongly, others see it as a sometimes tool.
P.S. I'm sorry this is an hour late, I got distracted playing around in GIMP.