It's funny you say that. I have astigmatism, but I learned to see these on demand before I got glasses to correct it. The tricks all involve getting your eyes to focus farther away than the actual image surface. I could easily see the butterfly while wearing my glasses. Then I read your comment and tried again without my glasses and I could still see something but it was no longer a cute butterfly. Without my glasses, it was an abyssal horror with wings unfolding from other wings in a vaguely butterfly configuration.
Do you need both eyes for this? One of my eyes is almost useless and also lazy, the other one has... Something, I don't recall what, but it's all blurry from up close and hard to focus. Guessing astigmatism. I don't wear glasses.
Yes, it's entirely dependent of tricking your eyes into changing their angle as if they are looking at something farther away than the image actually is.