It would have to be done by manually modifying the XHTML files to reference the same image in the EPUB images folder. There is no automated way (unlesss someone has developed a script, which I haven't heard about).
If InDesign EPUB export supported objects on a master page, you could do it that way, but everything on a master page is discarded on export.
Thanks. May I ask what the logic is behind the creating new image files in the epub for every usage of what is a single image, linked to repreated;y in InDesign? Obviously in InDesign, a user can change the source file of the link and have it update to everywhere it's used in the document--why wouldn't users want the same basic capability in an epub? I ask because Adobe engineers can obviously be counted on to have reasoned this stuff out pretty carefully, so I'm wondering if there's some underlying concept I'm not seeing.
We're not the engineers. I would suggest making a feature request. I think other matters had priority in fixing and enhancing EPUB in CS5.5 and CS6 timeframes.
This was actually an issue (bug?) that was introduced in CS6. If you unchecked "Preserve Appearance from Layout" on the Image tab of the Epub export dialog box in CS5.5 you would get one image that was linked multiple times. Now in CS6, unchecking that box doesn't seem to make a difference.
I use Find & Replace in Dreamweaver or Text Wrangler's (free) Multi-File Search to find the grep expression ChapterOpenerImage_fmt\d*.jpg and replace it with ChapterOpenerImage.jpg.
You will also need to clean up the content.opf file but they will all be together so it's easy to do that.
Hope they fix it. I think they will since it was correct in CS5.5.
Thanks! Yeah, I am hoping to avoid having to do that kind of editing and file-deleting and regex searches if I can avoid it. That would be nice.