Problem with quality of logo

Technical Details

Forum version : #phpBB2
Position : Administrator
Concerned browser(s) : Google Chrome
Who the problem concerns : Yourself
When the problem appeared : Today
Forum link : http://otaku-shrine.forumpolish.com/

Description of problem

Hello, i'm new here and i have small problem.

I make forum here with phpbb2, and i not get any problem with it. But if i make a small logo from the phone, i not have problems with quality. When i open computer in library i see small problem with quality of logo.

I need help right now! Thanks for help

There are two log/banner options:  one for main forum and one for mobile devices.