Hi - I think I went through the same issue and figured out a way to fix it but it pretty manual. So this will help if you list a few items a day or so, but if you have hundreds of items to list, this will not work. There is a site called canva, here is the link https://www.canva.com/youtube-channel-art/templates/
this site allows you to make free pictures for YouTube banners which allows you to make any picture smaller by just clicking on the edge of the pic and dragging it to your desired size. I think they let you upload, tweak, and download 4 pictures per month. But you can do more if you subscribe. Anyhow, once you drag your image to your desire size by eye sight, download the image and upload it to weebly and adjust accordingly (crop as necessary). That’s how I fixed this issue. Hope this helps.
