Just tried an experiment with an image. I flipped the image horizontally, then I changed her hair from blonde to black, and very slightly moved her arm, then changed the colour of the pattern on the outfit she was wearing, from a black pattern to a green pattern.
When I retried the reverse image search it came up with no matches anywhere, and the "similar images" were now quite different to the original ones, and were very heavily fooled by the amount of green, as the similar images were all heavily green weighted.
It turns out this was a little overkill - not having any idea what kind of algorithm the Google reverse image search uses, but knowing that it even finds images that have been cropped, I thought it would take a lot to fool it, and thought that just colour changes might not work, and so I moved the arm as well to significantly alter the layout of the pixels rather than just the colour of the pixels - but it turned out that redoing the reverse search after doing nothing more than the horizontal flip of the image, also made it unable to be found, yet all it was was the image in reverse. I'd have thought that an image search that can find a cropped version, should at least be able to find a flipped version.
My little experiment has basically convinced me that:
A) I do not have anywhere near the time to reverse search every image I own, so if any have been stolen I'll probably have to be very lucky to find out.
B) It takes almost no skill with Photoshop or even the most basic of editing software, to alter an image enough to make the reverse search no longer be able find it.
If you are planning on using the reverse search as your only protection for finding images someone may have pinched, it appears it may not be anywhere near enough to find it unless it's been taken by a person ignorant or foolish enough not to alter the image.
My biggest protection is that none of mine are yet worth stealing, but for those that do have images that people might want to take, don't rely too heavily on finding the theft by a reverse image search. It didn't take much to alter my image enough to hide it from the results.
I was actually really disappointed that it had taken so little (especially after all the effort I initially went to on the image to make it unfindable).