I have been playing with this for a while, and I tried different sizes with no improvement. Our group is set to a max of 2048x2048. My desktop is 1680x1050. My pictures have been resized to 1980x whatever so as not to eat up space. When I click on download the image is crystal clear. It is not my pictures.
I did find this old link where Mark says the pictures are displayed as 720x720 - https://beta.groups.io/g/main/message/2366?p=,,,20,0,0,0::relevance,,720,20,2,0,49528 and Duane made the following comment last year - https://groups.io/g/GroupManagersForum/message/8603?p=,,,20,0,0,0::relevance,,720,20,2,0,19578151
And yes it is an upsizing issue, but I think the pictures should be displayed full size when you click on it, or do a slideshow.
This is out a user's control. It has to be a change to the Photos code.