I've used PPT for 20 years and started using PowerPoint 2013 through Office 365 2 months ago and this has happened to me 4 times.
1. I create a presentation (Using copy and paste and/or insert to include pictures and other images)
2. After working on the presentation for a while I save the file and realize all the images (except smart art) are gone and replaced with the message above.
3. I have reinstalled PPT 2013 and it happened again. I have documents that are 50 pages long and have had to insert new images (Hours of lost work) then 3 days later it happens again.
Not sure if it is related but I've noticed that once in a while when I try to save a document (Happens in Word and Outlook Calendar items) it tells me the file is locked and makes me save under a new name.
This is really a show stopper. Any suggestions?