resolution of slider images

Hello, I am new to this forum and just started creating a website using the discover theme. The theme is absolutely great.
I have a problem with the slider images. The images look fine in the media library. I upload the images using “slider” in “theme appearances”. On the website the images look blurry as if the resolution is severely decreased. Anybody any idea how to solve this?
Thank you.
website: http://www.stellarproject.eu/