No, I think. As long as 20% of the picture is different, it doesn't matter how you do it. Adding a black box in one or more corners is a simple way to do this.
Sounds good. Can also change colors, color saturation, add border on all sizes of different thicknesses, colors, and shapes (wavy lines).
Remember that the ideal size for twatter images is twice as wide as it it high. This leaves a lot of room on left and right side to switch out all kinds of squareas and other images.