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?
With a standard Licensed image you are allowed to modify the images you buy so yes you can change the backpack to your own.
And yes you are allowed to use the modified asset in your online shop to promote the 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.