Unsharp images in portfolio section

Hi all,
that has probably been asked previously, but I can not find the anser. In my portfolio section I find it difficult to get sharp images in portrait format. The full screen images seem to work ok-ish if saved as 1920 x 1080 pix. However, if I create a photo in portrait mode (not landscape) and use 1080pix x 764 (hight x width) I always get unsharp pictures. I also tried changing the image height to 768pix or 1500pix.

I tried deactivating the optimise plugin, and changed the settings in the media library to full-size, but it makes no difference. Could you please let me know to what size/ height an image in portrait format needs to have, or if I need to change any other settings / files to make sure my images are sharp. I’m an illustrator working with photos, so great pictures are of upmost importance.
Your help would be very much appreciated. Thank you ever so much.
Jacqueline