Images are not technically inserted into an HTML page, images are linked to HTML pages. The <img> tag creates a holding space for the referenced image.
While the content of the <figure> element is related to the main flow, its position is independent of the main flow, and if removed it should not affect the flow of the document
From http://dev.w3.org/
“The img element represents an image,” while “[t]he figure element represents a unit of content, optionally with a caption, that is self-contained, that is typically referenced as a single unit from the main flow of the document, and that can be moved away from the main flow of the document without affecting the document’s meaning.”