Finally got the phone backdrop image thingy working again. The problem was that uploading the picture to tilde.town takes a finite time and if I triggered the change again during that time, Tasker would sometimes lock up. To be honest, I haven't solved the problem but I've decided to live with it for now.

BTW Tasker scales the picture to fit my phone's screen but that doesn't happen on the website so the images vary a lot in size.

BTBTW Tasker couldn't do all this on it's own, I'm also using commands from the AutoTools plugin as well.