Some images are not visible in Power point - but I can see them on another PC with Same ver of Power point

Hello,
I just observed, that my Outlook sometimes doesn't show picture attachements.
And they are coming from the handy of my sister.
But havn't put effort into it until now to find the reason. 
 
So it is possibly worth to inspect the pictures, where they are coming from and if there is any connection.
Inside Power Point, you may have a look, if they are embedded (so inside the file) or just linked.
Linked pictures are stored somewhere else and need access from the user side.

Next option is, users like to just put they fotos from the cam into such application and forget, that the sceen resultion is much smaller than the picture. So possibly a lot of rendering work on the client.  
The rendering need some system resources and the rendering has to reconize what is inside.

You talk about "some" pictures. So maybe worth to take one of them, use a tool to make them possibly a bit smaler or change anything else and save it again. If this picture is then visible, I would assume that there are some data / metadata information inside the picture, PP has problems to interpret.    
Interesting, if the affected client can also open the original picture. 

If you're getting a red or blue x, that's one issue, but if they are just not showing up, probably it's a memory issue on the computer in question. You may just need to reboot and or shut down other applications that are running. I know I occasionally run into weirdness like this when I haven't rebooted for a couple of days. 

Normally, PowerPoint images are embedded. But they can also be linked, which will cause them to disappear if they aren't in the expected location on a different computer. If you choose File>Info and there is an Edit Links to Files notice below the Properties, this is probably what's happening.

You mentioned the same version of PowerPoint. To some people, that means they are both PowerPoint 2019, but could be at different update levels. Some updates of PowerPoint have issues with disappearing images, so it's worth checking the complete version number using File>Account and reading the About PowerPoint section.

The user get the handles of an image thats pasted into power point to word or Outlook.
So handles only.

If exported to .pdf it can be seen

Maybe it is worth to run the task manager in the background, just to make sure, the computer doesn't run out of ressources. 

Sounds like a memory issue to me.

Also, have the recipient save the file to their harddrive rather than opening from Outlook or an online storage.