It looks to me like it is necessary that images that are to be included in a signature must first be posted on a web site so that you then link by
citing a web address (URL). I can't see a way to
upload an image file to become part of the signature.
You can upload an image file to be included in a posting, just not in the signature that is automatically applied to every posting.
Some web sites
allow you to link images posted on other web sites into postings in addition to uploading an image file directly to the site. My guess is that this is supposed to be a convenience for those who have images posted elsewhere, and don't want to have to go through the hassle of uploading them again (every site seem to have its own set of limitation of file type and file size - there is no 'one size fits all'). But it doesn't always work. For example, the Large Format Photography site (
https://www.largeformatphotography.info/forum) offers that option, and my experience is that many of the images that are linked from other sites are not visible because the link to the site where they are posted in broken, or because my browser can't handle such 'second-hand' postings.
And there are even a few sites that do not allow posters to upload images, and require that if you want to include an image, it must be posted to some other site. That's a real PITA.