Like
How to change group logo and group cover quality?
Because group cover and logo have very poor quality
Maybe the over was loaded a small photo?
Try larger better quality.
Like Paris said, change the size of the image you are uploading since both the logo and the cover image is hardcoded to fit which users need to know for cover images. The logo area is small so it looks crappy if you have a website logo bigger than 6 letters like phpfox. Check this thread out and hopefully it can help.
https://community.phpfox.com/forum/thread/58001/change-the-logo-size/