How do I get the Black bars off my artwork

[MaybeCan]

Hello, I wanted to make a custom profile with an artwork, but unfortunately my artwork doesn't fill the entire area and there are side edges that I actually want to have filled with the image.

https://prnt.sc/OikbDD6_Ojmn

If someone doesn't want to click on the link, feel free to look at the whole thing on my profile.

[N3tRunn3r]

https://steam.design/

https://steamcommunity.com/sharedfiles/filedetails/?id=748624905

There are some tutorials and editors..

Back in the days you had to use a hack applied via browser, this guide linked above clarifies.