I am fairly new to Groups.io and have the same 'issue'.? It doesn't matter what size the photo is when it is uploaded, it is blurry on a desktop screen (it is fine on a phone).?
The image is sent to your browser at a maximum of 720 x 720 for display on the photo page, but is retained at the uploaded resolution for download at full resolution.
How "blurry" that will appear will depend on how large a display window you have, and maybe the characteristics of your browser and the file type of the original image. I've not noticed much of a problem with normal jpg images, but your mileage may vary.
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