The hook marked in green has a ribbed top edge.

Adjust the position of the picture so the nail is located at one of the indentations, so it hangs level.

If it droops one way on a particular groove, and the other way on the next groove, then you need to counterbalance the picture.

You can do this by attaching a weight, such as a clean nail, to one of the clips on the left or the right of the picture, perhaps with cotton or string, which can be removed later.
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*Maybe the pictures really are level, but their subject matter creates the illusion that they are crooked.*