impress version 4.1.5.3_de: photos vanish unpredictably

Hi,
I have build an impress-document with sound and photos (28 slides with a maximum of 4 pictures per slide) for a family celebration. I imported the photos per drag and drop and used the same laptop as well for building as for presentation. Unfortunately during my presentation in public some of the included photos simply vanished! The slide was there, the sound was on but no photo! After a break I reloaded the vanished photos an tried to put it in as a “Link”. That worked better but still in some unpredictable cases some photos still vanish during one presentation and after restarting the presentation they reappear???
Does anyone have a clue?
Thank’s for Your reply!
Uwe
I use Win7 Home Premium, SP1, Ger

What kind of data format did/do the disappearing photos have?