Hi,
What kind of file format for the image did you use?
I suggest you refer to this thread about the similar question:
https://answers.microsoft.com/en-us/msoffice/forum/all/macos-how-to-stop-image-compression-in-powerpoint/3f0181d8-66ac-4292-a0cb-455a247ae8bb?page=3
It seems there is no "Do not compress images in file" option for PowerPoint mac.
I suggest you check if you can find the Compress Option in PowerPoint for Mac and set the highest size:

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