Hi @Bölling, Daniel ,
You mentioned that OWA can be used in its entirety. Did you mean that after you add it locally and sync to the cloud, you can see the complete photo after logging in to OWA by your on-premises Exchange mailbox?
Please note that when uploading pictures from local AD, The Active Directory thumbnailPhoto attribute value is limited to about 100KB in size – this will mostly prevent you from uploading a “high resolution” photo. And Exchange Online accepts only a photo that's no larger than 10 KB from Azure AD. If you use this method to synchronize photos, 96x96 is more recommended.
When we upload larger pictures, Exchange will automatically crop these pictures. Although there is no official article that points out a specific value, we can know from the following official article that it will automatically adjust to three different photo sizes and resolutions. According to my test, when the 648x648 prior image is uploaded directly to Office 365, it will be successfully applied. So I think it is the limitation in the synchronization process that changed the image you uploaded. It is more recommended that you upload the pictures directly to Offfice 365.
For more information: Configure the use of high-resolution photos in Skype for Business Server
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