It would be great if there was a canvas option to make the image square by increasing the canvas size. Whichever dimension is largest, increase the other dimension to that size (along with current alignment and offset options). This way a 400x500 image becomes 500x500, and a 345x230 image becomes 345x345.
Never underestimate the power of Phatch.
Please set both width and height in the canvas action to:
and choose pixels (px) of course
This will do exactly what you want. You can also choose min instead of max.
Fantastic, that works quite well. I think that adding a checkbox to "Make canvas square" would be useful, "greying out" the width and height options (make them not available while checkbox is selected).