Design mockup infringing copyright? - 1

I use these blank slate images for these design mock-ups, like billboards and bus posters. A few have been supplied by my lecturer during this website assignment, and he made it very clear we can use them for free and things. Recently, I downloaded a free blank slate image for ipad to put a website mockup on. And over all these blank slates, I've been putting watermarks on all of them. Am I allowed to put a watermark on them? I don't want to infringe. If I am, and someone steals that image, have they stolen from me (ie. Watermark on it), or stolen from the creator of the blank slates? Keep in mind that I don't intend to resell these blank slates, these are just for demonstrative purposes only.
Added (1). I've been trying to find the license to it, but it's not available at all. It just says free to use.
Here is the website if it makes anything clearer:
http://www.oxygenna.com/freebies/iphone-ipad-photorealistic-mockups#

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