This is about putting images in your Description and not the 12 pictures you are allowed when you list?
Those 12 are the first thing you are told to upload when you start the Sell Your Item form.
Personally, I scan my items (stamps are flat) to my desktop then just drag and drop them onto the form.
They can be manipulated a little once there.
Is that what you are asking?
If it is putting pictures into your Description using HTML, I know I've done it in the past when eBay charged for pictures, but today I only use HTML to make the Description in my listings show up better amid the clutter of other ads etc.
I found this useful for a techodolt like me when I started out.
https://www.tutorialspoint.com/html/html_formatting.htm