Image size in Single Tour

Hi all,
I uploaded some own pictures to the image gallery of a tour product. Upon upload, the image is resized to different sizes, which is I think normal behaviour in WordPress. When displaying the single tour on a desktop, the images used in the picture gallery are the ones resized to 600×400 px. However, due to the size of the monitor on a desktop, the actual size of the displayed image is 812x609px, meaning the image is scaled up which results in poor quality of the picture.
Can you tell me whether there is a config option (or custom css option) to either make the system use a larger version of the picture for desktop purposes or restrict the size of the displayed images to 600x400px?
On mobile, the quality is fine as the pictures are not scaled up.
Thanks!