URL - universal resource locator, sometimes referred to a "EARL" by us geekies. That is in essence a web address as you would see it in the Address bar of your browser.
HTML - Hyper Text Mark-up Language. That is the basic framework of tags that are a web page. Tags can do everything from denote the header of the webpage to the font (and what properties are attached to it such as bold, italics, size, color, etc...), to even telling the browser where 'scripts' begin and end.
IMG - thats one of the html tags. IMG referrs tells the browser that an image belongs in that space and what its properties are (size, width, location, etc.).
Does this answer the original question?