On his other photos, Andy seem to be in the habit of using Scotch Tape, either the double-sided variety or perhaps just folded over, in only the top corners.
However, on his Raquel poster he also uses tape on one side, but not the other.
Based on what we can see (no visible tape at the top and a single piece of tape visible on the side, that may or may not be stuck to the wall) it appears that the poster is attached securely only at the top and that when he digs, Andy carefully lifts it, then allows it to fall back into place. This general wear and tear would also explain why he needs to replace the poster at least once.
When Andy made his escape, he presumably lifted the poster (to allow him to climb through) and let it drop back into position under its own weight.
In the interests of science, I've just performed a simple experiment, hanging a piece of glossy paper to a wall by a single piece of sellotape at the top, then throwing a stone at it. As you can see, the stone easily passed through the paper, without any need to secure it at the four corners. It had plenty of "taughtness" all on its own.