I am working on Divi 2.5 version ( current legacy version is Divi 2.3 still ). I found a troubleshoot for the unclier images problem.
Find the lines inside
$width = (int) apply_filters( 'et_pb_portfolio_single_image_width', 1080 ); $height = (int) apply_filters( 'et_pb_portfolio_single_image_height', 9999 );
and comment these lines.
Else you may get very unclear thumbnails, based on the original knowing how WordPress creates these.