WOW, that's a lot of information!! Thanks, Othesus!
I don't know a thing about the different modes, but I noticed the "beacon effect" on my own rings when there were non-transparent pixels too close to the edge of the image. To keep this from happening, I've been working with a 512x512 canvas, but the opaque layers are no larger than 500x500 (6-pixel transparent "no man's land" on each side). It seems to work so far; I haven't encountered a "beacon" since I started using those dimensions (EDIT: and choosing "No MIP maps" when saving).
For my save settings, I've been using the defaults in the image I posted in the OP, but I set my "Save Mode" to "32 bit ARGB" (which I assume translates to the "8:8:8:8 ARGB" format that everyone else seems to find worthwhile) and my "MIP Maps" to "No MIP maps."
It seems to be working well for me so far...I've released a
Stained Glass and an
Aztec Sun target ring already, and I've got an Art Deco one on the way.
Thanks so much for all those links. I have the distinct feeling that I'll be referencing this post a lot in the coming weeks, LOL.