MDN is my favorite resource outside of stackoverflow. This article may be helpful.
Without seeing your code it’s hard to say what the issue might be. Your ‘src’ attribute should be a link to a valid image. A good test if your link is a valid image … try opening it in a web browser. If your browser contains only the image then you’ve got a good link.
keep in mind “This [the image tag] is an empty element (meaning that it has no text content or closing tag)”