I suppose that any sort of media file hashed would be a relatively good source of random bits, but I'm not an expert in this topic. If it were me, I'd be sure to either use a whole file or some portion of it which is definitely not a header (first bytes of many filetypes are going to be the same for every file of that type).
Don't use a picture that is already posted on the internet, and don't use a song from the top charts. (actually, don't use any song unless you are sure you are the only person who recorded it.)
Definitely don't use a recent movie even if it never won a grammy award.
But just in case you do, make sure your equipment goes through an analog portion. So there is some noise introduced.
I tell you what I will do when I get my new DSLR, is I'm going to go around town and take pictures until the memory card is full. Copy those files to an offline computer. Then hash each one of them.
To be hard core, use the RAW format of your camera, if it is available, or the highest resolution.
Good luck with anyone figuring out 24 megapixels of data.