What is the main difference between these photos?

This is my interpretation of this assignment.

It took me along while to come up with a nice idea for what I could do that would be subtle, yet easy enough to spot. When creating this piece I was constrained to items on my desk. I thought it would be pretty cool to use the wood pen holder I have, because it kind of looks like a tree trunk if you can’t see the top (which has the holes for the pens). This assignment took me a lot longer than I thought it would, just because I kept fiddling with all of the different positions of the items on my desk, trying to make it look good. Looking back on this assignment, I wish I had gone outside and tried to do this with something more natural, maybe trees or something along those lines. It would have been interesting to perhaps remove a branch in the back of a small tree, or something along those lines. Although this would have been probably impracticable.

The answer to the puzzle: The cap on the left has been rotated to the extent that the text inside the cap is visible.