And thanks to a suggestion from another Etsian, I right clicked on one of the older displaying images to get the properties. It shows the URL and the image dimensions at 575 x 575 pixels. Images prior to about mid March were 155 x 125 pixels -- which were easier to scroll through.
Then I tried right clicking on the tiny sliver (that looks like a line) of the non-displaying photos in recent listings. If I place the cursor in exactly the right spot, I can get the URL and image dimensions of 575 x 1 pixel.
So the images are there, but are displaying at only 1 pixel high in IE's RSS reader.
Etsy, you need to fix this. I'm not the only one experiencing this problem, and regardless of whether the images may display in other readers or browsers, it also needs to be functional for users of IE.