How does Andy Dufresne reattach the poster to the wall in Shawshank Redemption?

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.

If he weighted the corners at the bottom with washers taped to the inside corners they'd stay in place as if they were stuck to the wall. Elementary my dear Watson's or should I say Shawshakston's?

It's not clear but assumed it was held up on the top by common tape. The poster was flat through hanging there a while and thick enough to remain flat. Along with him rolling it and temporarily securing it would keep it from getting damaged while he tunneled night after night.

It seemed it was quite a long hole so he would have to physically enter it to complete it as the exit was far out of reach.

I wondered how he fit through it. But he did.

Also, the poster may have been rolled in the opposite direction to curve inward toward the wall. I speculate it was not secured to the bottom to begin with the curl of the paper had long since left the poster. If he repeatedly replaced the bottom tape after each tunneling session it would've appeared odd for cell inspection so he may have moistened or dampened the wall when finished to get it to adhere but not permanently stick to the wall.

Other possibilities were the vacuum pull created by the hole once he was through held the poster secure to the wall. If you watch as the warden tears the poster from the wall it rips away pretty easily not as if it was taped on the bottom.

Another way would be to coat the back of the paper with a mixture of the wall debris and something sticky to make it flat eggs and water would even work.

Perhaps Andy had stuck two small magnets on either lower corner on the back of the poster and two small iron pieces glued to the wall that would hold the magnets in place and keep the poster taut.

It would not be a big deal for Andy to get two little magnets and two small iron pieces from the prison's machine shop.