Some additional information:
The problem lies in the CSS style _idContainer003
which has these attributes:
When I remove that DIV tag from the ePub file, the image displays perfectly. When I change the Width attribute to 100% from 42.61% the image displays just fine.
The image itself is not sized.
I can make the change to the CSS in the final file, I guess but I'm wondering if anyone else has the same problem.
Does anyone know where that DIV tag comes from?
Figured this out on my own but leaving it here because it was such a FRUSTRATING problem!
My image's object frame was not sized at 100% across the entire width of the text frame where the image was anchored.
I don't know why the image disappeared when it wasn't scaled, but I know that when I fixed the width attribute for the container in CSS, it worked. Then I figured out that the value of the width attribute was getting set by the size of the frame. When I resized it to be 100% it worked.
My key takeaway from all of this:
If your images are not showing up in your ePub files after exporting from InDesign, first crack open the ePUB file and remove the surrounding CSS. Then repackage as an ePub.
If the image displays, the problem is in the CSS.
Experiment with the original CSS to find the right settings that will eliminate the problem.
Finally, see if it's possible to work with InDesign to get the right settings to appear when it exports. If not, at least you just have to change the CSS file, instead of changing every individual IMG tag in the XHTML files.