Disclaimer: the following is not legal advice. You should contact a lawyer if you want legal advice.
The reason you can't use stock photographs for logos is because when you buy a stock photograph, it doesn't grant you copyright - only a licence to use the image. Stock photographs can be sold repeatedly, for anyone to use.
Logos on the other hand should be original creations which are copyrightable. If a logo were to contain a stock image, it wouldn't be copyrightable since you don't own the copyright. You could never claim that logo as your own.
Perhaps your best bet is to take your own original photograph, or hire a photographer to take a photograph specifically for your own exclusive use, and who is prepared to sell you the copyright - which would mean nobody else could use it (not even the photographer).