When the user copy-pastes an image from Outlook, it is pasted as an IncludePictureField and visualized as expected. The field code looks like

{INCLUDEPICTURE :image001.png@01D15D16.4FAF0970 \* MERGEFORMATINET}

However, when the fields in the document are updated, the image is no longer shown because of its invalid Uri. In MS Word, when you paste the image, it is directly inserted as image and the field is skipped.

Workaround - remove the field start/ends (in the attachments)