Did you follow the directions from HT5585? HT5082 is for 10.7 not 10.8. Look at the bottom of the HT5585 document under Copying all wikis from one OS X server to another OS X server.
I suspect you took care of the postgres backup but might have missed copying the contents of /Library/Server/Wiki/FileData, of course this assumes that all the images are missing and not just some. When I migrated from 10.6 to 10.8 wiki a number of images vanished, but most attachments and pictures survived.
Hope this helps.