There are all kinds of tricks webmasters use to try to protect images from theft. The latest I've seen is a trick where they slice a single image into nine pieces like a sloppily drawn tic-tac-toe board (the spacing of the lines is random.) They then use HTML to reassemble the slices but if a user tries to right click to download or search them then they just get one piece of the image.
Really, watermarks are the only thing that really helps with theft because as UncleBob said, you can always grab a screen shot and then either search for it or reuse it on Backpage. That trick would work with the new "nine slice" trick as well. It's not something everyone knows how to do but any web designer or programmer would. Any image you allow a web user to see can be captured.
That said, I also find the watermarks annoying and I don't know that they are all that important - at least not from my perspective. In Montreal, you're lucky to have a handful of reputable agencies and indys along with this board to verify authenticity. With that combo, pictures are likely to be authentic. The worst thing I've had happen is that the pix were out of date and the girl had changed her look a bit. (Actually, the worst thing was a bait and switch long ago but that's a different story.)
In other cities (for me, mostly NYC) you have to assume pictures are fake until you can prove otherwise, and in those places that proof is harder to come by. There is the general escort review board that helps, but it's not nearly as simple to use as this board and often it doesn't clearly answer the question. It's always a bit of a risk.
So, going back to Booker's original question - the alternate format could be an innocent/amateur attempt to protect images, or it could be just what you suspect - trying to block anyone from checking. My policy is to look for in-person verification. In Montreal, I won't use an agency or see a girl without it - there are too many well-proven options.
Booker - if you're interested in checking a specific image where Google lookup is failing you can PM me and I can give it a try using the trick UncleBob suggested.