I would like to make sure if I understand correctly. My question is:
If I buy a standard licneced image of a man with a backpack and remove the backpack and replace it (in Photoshop) with one of my own backpack designs is it allowed? And am I allowed to use the new composite in my on line shop to show off my backpack?
your use is perfectly in-line with the license terms.
To add to mbglobetrotter correct answer: That's the escence of stock images.You buy a picture, modify it to your needs and use it. The only restriction is that you should not represent the subjects in a degrading situation, like the use on adult sites or for p*rn*graphic uses.
Regards, Abambo Hard- and Software Engineer and Photographer.