They are all already .PNG files, so no luck there. I assume that Opus is hard-coded to only display images up to 128x128, though I can't work out if there's logic or pot luck behind whether your icon gets scaled to 128 pixels on its longest side, or 50% size.
I've just spent a few minutes aligning my toolbars very precisely (to the pixel - a lot of time spent staring very closely at the screen) only to have the old resizing toolbars bug crop up again (I've posted about it before). Also very frustrating. I'll have to try and trick the bars into staying put before I align them all over again.