I will start off by stating that I am well aware that this feature would involve major work to integrate to mIRC, so I doubt it would ever make it into mIRC. I can hope though.
I would love a feature like the browser extension HoverZoom for previewing image links without having to click the actual link. I used a browser based client the other day, and being able to just hover over a link to view images was an amazing experience in terms of usability. I know mIRC would need to have some sort of tie in to a browser engine in order to make this work, but it would make my day if I didn't have to click every single link just to see pictures.
I did some quick searching on the web, and I couldn't find any non-browser based clients that will do this feature.
Also, thanks for making an amazing IRC client. I've been using it since 1995, and I do have a legit paid version. mIRC for ever.