Hi @Christina Maas ,
According to my research, this issue seems to occur when the original message has been sent with a non-HTML format, such as Rich Text. There is no built-in feature in the Outlook client to always reply to all messages in HTML format, so you may need to change the format manually.
Another workaround is to reply in HTML format automatically with VBA, you can follow the steps in this link: https://superuser.com/questions/1674962/prevent-outlook-from-replacing-images-with-text-ole-object-picture-device (Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)
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