I feel that it's a difficult site to monetize. Part of the appeal is that it can be more convenient than having a resizing application on your computer. I think adding sign in (a dependency for premium plans) would undermine that a bit.

At the moment, it's still growing quickly, doesn't take up much of my time and costs barely anything to run. Given those circumstances, I don't feel I need to rush into monetizing. I'd rather wait and see.

Probably the best thing that could happen in the meantime is widespread adoption of a very low friction micropayments platform. I think PayPal or bitcoin are best positioned for that, but I haven't seen any signs of that ideal scenario coming to fruition any time soon.