OK, so I will have to resize it to a power of two...
hmm. I think it will end up meaning that I have to remake my background, and some other objects in my game as I am using a tiled perfect hexagon texture which happens to be impossible to do in a square form.
So will there ever be a solution or is this just a limitation of WebGL?
EDIT: I managed to get it in a square, 512 by 512 pixels. it worked, but my game is lagging a bit now, could it be thanks to the large tiled background?