It's a long-standing design term that's come across from print to the web. You could just as easily call it a "promo" (promotional) image, a primary image, a main banner, etc.
Most would use IMG as it should theoretically be actual content and not just decoration, so you'd want the ability to manage it via a CMS and to add alt text.
Broadly though if this really annoys you just make your own and call it something else ;)