I could see what i.php is doing, but only from a distance. If I tried to mess about with that, it would be nothing but a pig's breakfast. Altering local/rules.css is easy though.

What I wound up adding was:

/*for individual photo thumbnails*/
.content UL.thumbnails SPAN.wrap2 {
  box-shadow:3px 3px 3px rgba(0,0,0,0.8);
  border-radius: 3px 3px; /* round corners with CSS3 compliant browsers */
}

/*for category thumbnails*/
.thumbnailCategory img {
  box-shadow:3px 3px 3px rgba(0,0,0,0.8);
  border-radius: 3px 3px; 
}

*And* this saves me the trouble of making thumbnails for every upload and being sure the filenames are okay! Whee!

I am a very happy camper. Thank you all!