This is a feature request for E-mail clients that want to use the HTML widget for viewing HTML messages with inline attached images. When the HTML component requests the images of a IMG tag (in an E-mail), and I don't yet have it, then on a modern IMAP server I can request a CONVERT-ed version of it, that is adapted to the screen's size for example. So if the HTML component can offer me the ideal size for the image, on a modern IMAP server that is very useful info because I can request an image that has its size reduced. This saves bandwidth on a device that is using GPRS (GPRS's bandwidth is expensive stuff that you don't want to consume just for the fun of it. Unless you want to anger your user)
I think the right way to do this in the WebKit2 world is to modify the HTML as it is displayed, so it's possible to do this without special API.