This may sound like it's not a problem, however due to the default keybind being set to "C", it also breaks any other keybinds (such as zoom) that may be set to C.

I think you should enable debug mode in the PictureSign settings and restart the game.

Unfortunately I'm running into this as well. I don't use MultiMC, but anything that I bind to C doesn't work when I have PictureSign installed. This includes when it is set to Debug mode.

Removed in later versions of the mod.
In older versions, you can change the keybinding after enabling debug mode in the config and restarting your game.