The attached GLBasic project creates all assets required for your STEAM page.
You just replace the files "banner.png", "logo.png" and "font.png" with your stuff and in the source change the app$.
It will proportionally scale the banner background, add the logo at the left side and scale the app name to the right side for the capsule graphics.
That's handy, ain't it!?
Updated. Uses ReSampler lib for high quality images and a DDgui.