If you don't own them you can't use them!
If you find an image (or text) in a book do you think you have permission to use it without prior authorisation? Rhetorical question - of course the answer is a loud and emphatic "no"!
However, writing to the relevant site webmaster may prove fruitful in any given case where you would like permission to reproduce given material.
Basically, just because an image is not watermarked does not mean that you have the intellectual property to it!
Hope this helps :-)