As a user, I much prefer a blocking UI thread to one that lets me spam clicks on the "rotate left" and "flip along vertical axis" buttons and then makes me wonder why the resulting image is not the flipped verstion of what I saw the moment when I clicked "flip". However, I do like being able to abort my operations, and a blocking thread does not let me do that.

It might be quite a hard problem to determine which buttons should be disabled during which operations. One tricky candidate is the "safe" button. Should I be able to click it when the visual feedback I'm getting does not yet match the internal state of the application (which is what will be saved)? Should I be able to start further tasks when the save is still in progress? (If so, what if the save fails? Will I be able to roll back to the state before the attempted save and try again?)