First of all, thank you very much for your fast answer.
We failed to reproduce the issue by ourselves. It happens to different colleagues who works on Mojave, Catalina, High Sierra. Every body work on Ma. The version of MacOS does not seem to matter. It happens quite frequently, like 1 times a week. But it is not systematic, I mean it does not happen for all files, any times. Extension of files is .pptx and it also happenned once on word (.docx). We work on Microsoft 2019. However, we notice that often the steps to get this issue is : a user save the file, then a 2nd user open this file quite quickly after it has been saved by the first user, he can see pictures, save the file and by then the first user open again the file and cannot NOT see the pictures anymore. It is not always in the same order, but it is each time this kind of situation. An other thing is that sometimes, once pictures disappeared, when we open an older version of the file to get pictures back that are not anymore in older version of the file either.
We run Sophos Antivirus on Mac.
Do not hesitate to ask further information, I really need help.
Thanks in advance,