I chose an image from a regular upload directory and it works fine. I chose one from an S3 source set up in Assets, and it doesn't. For instance:
<img alt="" src="http://s3.amazonaws.com/foursquare.org//1350928362-IMG_3335.PNG" />
The double slash is causing the image not to display.
I'm using WYGWAM 2.7.1, EE 2.6.1, Assets 2.1.4, and a lot of Scotch at this point.