To change your background profile image, view your profile and upload any JPG or PNG image over 1920x640px.
We are often asked by users how to change the profile background image to something that is thin, as uploading a 1920x640px image causes your portfolio to be pushed down the page.
ArtStation is a responsive site - meaning that it is viewed on a variety of devices from tablets to huge monitors with different aspect ratios. This makes our handling of images more complex.
The minimum size required to make your profile header image look good (no upsizing) is 640px height. As long as you upload anything larger than 1920x640, it will look good and will responsively resize the image, retaining the correct aspect ratio.
To get a thinner image, upload something with a larger aspect ratio. E.g. 2560x640px.