How on earth does one Select All and then apply a 1px black border? And why isn’t it completely obvious to even the most casual observer on earth? Every simple thing I try to do in Acorn turns into a Help question. (OK, exaggerating for effect). Thanks in advance.
The way to do this in Acorn would be to add a rectangle shape, and move its corners to the edges of the image (it should then snap into position). You then use the shape inspector to turn off fill, set the color of the border, and then set its width. In this case you’ll want 2px, since half of it will be drawn outside of the canvas.
There’s also an “Add Border with Options” plugin you can use: https://flyingmeat.com/acorn/jstalk/AddBorderWithOptions.jstalk