ID:278215
 
For instance - I have a picture I saved (from imageshack) on my hard-drive. Is it possible to trace where I got it from, or would people have to rely on me telling them/guessing?
There's really no definitive source information in the metadata for images, unless someone specifically puts it there.
There are several image based search engines that let you upload a sample to find. I don't know how well it works, but people have mentioned tineye.com.