As such, Stray end tag... means just that an end tag is not allowed in the context where it appears. As the validators explanation says: The Validator found an end tag for the above element, but that element is not currently open. This is often caused by a leftover end tag from an element that was removed during editing, or by an implicitly closed element (if you have an error related to an element being used where it is not allowed, this is almost certainly the case). In the latter case this error will disappear as soon as you fix the original problem.
From the symptoms (the error message string), we can infer that you are validating against HTML5 in HTML serialization. This means that no end tag is allowed for an img element, since the start tag is treated as also closing the element (implicitly closed element).
Thus, the solution is to either remove the </img> tag or to validate against HTML5 in XHTML serialization. The latter is not practical for web pages, but if you are using HTML for something else, you should validate by URL referring to a resource that is served with an XML content type.