you'd have to specify your question further - are you using webmail & which one (the functionality will depend on the question if your provider has implemented it properly) - firefox in principle is capable of doing it) or do you want to get images into a desktop mail client like thunderbird or outlook?

This is regarding webmail.

apparently yahoo once offered a plugin to achieve this (but as far as i know they haven't kept it up to date so it won't work in current firefox versions). other webmail providers like gmail have integrated this feature out of the box!