I found that when I dragged a photo from Free Commander (better than Windows Explorer and available in a portable version) onto an Impress presentation the photo would disappear if I later moved the photo to another folder or deleted it. What happened is that Impress created a link to the original file, so if I edited the photo the new version would automatically appear in my Impress presentation. This works well until you move or delete the original file.
You can prevent the photos from disappearing by breaking the link. Click Edit>Links (if Links is grayed out your presentation doesn’t have any), then click on the link that you want to break. If you want to break them all click on the top one then hold down Shift before clicking on the bottom one. Finally, click Break Link. Now all your photos and other linked files will stay in your presentation unless you open Impress and remove them yourself.
There is a way to prevent the automatic creation of links. In FastStone Viewer (portable version) I go to a full screen view of the photo that I want to add to my presentation. Then right click>Copy Image. Finally, I go to the relevant slide in Impress and right click>Paste or Ctrl+V.
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