Profile picture size

Hello , i have one problem/question to ask about your theme. Basically i am working to customize the theme so it will correspond to my wishes . I wanted to make the background picture from a user/team profile bigger , to change in main.css from 1920x280px to 320px . To make it more clear i will insert here the part of the code that i modified :
Team Profile main.css.`line 4878 .profile-fimage.profile-media-team .hiddenoverflow {
height: 320px;`
User profile main.css. : `line 5053 .profile-fimage.profile-media .hiddenoverflow {
height: 320px; `

So , i wanted to make the pictures bigger in the both situations that’s why i modified those lines , but , there is always a but, now i have a problem. After i upload a picture it’s all good but when i hit f5 and i wait till the page reloads the image doesn’t look clear anymore , it’s loses quality.The problem occurred even when it was in the original form , before i changed the px from css.

Can you give be a piece of advice and help me solve this problem?
Thank you , i am looking forward for your answer.