One of the regular offenders for not giving credit for images is the Facebook page The Doctor Who Hub (formerly known as Doctor Who and the Tardis by Craig Hurle). Not too long ago, the page shared a photo of an awesome sonic screwdriver box and made it sound like it was too hard to find where it came from:
I right clicked on the image and chose "Search Google for this image" and discovered the initial post by RPF user 00failure in less than two minutes. I commented on the page to point out where the image came from, and I was surprised and kind of saddened to see that none of the other commenters (and there were many) seemed to care. It made me wonder if people realize how easy it can be to trace a picture back to its source.
The Google search is almost always the solution. It can get tricky if the search only returns results from Tumblr because Google often links to the general Tumblr account rather than a specific post, but if you search the Tumblr account or look at the archives, you can typically find the beginning of the trail before it was reblogged. The more widespread the image, the harder it is, but more often than not, I can track down the source and credit it.
Memes are the hardest to pinpoint, but again, not impossible. Sure, playing detective takes a little more time, but it's only fair to give credit (and link to it) whenever you can. Because really, it's not that hard.