What is the Syntax to add image from WordPress Upload Folder?

I am trying to include into a Loop a custom icon from the Wordpress’s upload folder. It isn’t a field, it is before a link to download.
Either as CSS class, so:
li.icon1 :before { background: url(...); /* Other parameters */ }
or as an image with its own CSS .class img and into Loop:
<img src="https://domain.com/wp-content/uploads/my-icon-qr.png" alt="Download QR image" width="16">
It works padding, display and all but shows the space or in blank (in :before) or if image wasn’t in this URL (as img scr).
I’m sure my syntax is wrong. You can help?