Hi @niko585 ,
The good banner size for these days is 1920px X 1080px (Full Screen Presentation).
For the mobile banner size is 480px X 480px (but I use 840px X 840px for mobile & tablet).
If you spend a bit more time editing the images (for sharpening & brightness adjustment), you can make the images better.
Did you try to photoshop that image? Maybe it good after that
Hope it will help you!