Sometimes, the panel only has certain elements incorporated into it, such as a face, background, or posture. Other times, the entire panel has been taken with just certain parts changed.
There are times when this is a good thing, such as one artist paying homage to another. Other times, the artist sees something they like and tries to incorporate it into their work. Then there are some artists that trace their own previous work to save time in the creation of a comic book page.
Most camps of comic book fans frown on this practice, especially when the original work is uncredited, and the artist in question can come under further scrutiny and even legal action.