If I choose the rectangle selection tool, I can click and drag, and once I have made a selection I can resize it. For both these verbs, if I hold the "shift" key, the selection snaps to be square.

An option many art tools have, but pinta does not, is to hold down a modifier key and force the selection to "center". For example, holding down alt while either drawing a selection, or resizing an existing selection, could cause the opposing handle to mirror the motion of the drag so that the center (the drag start point for a new selection, or the center of the existing selection for a modified selection) remains unchanged.

This would be very handy when using Pinta for cropping, because it would allow you to eyeball reducing the size of the selection without de-centering the existing content.