Any 'thumbnail' you've ever seen is 1 of 2 types:
1. The image you see has already been resized so you can download it quickly
2. Your computer shows you the image in smaller dimensions, its still the entire image and uses as much system memory and HD space, its just shown smaller on your screen.
On the web you are likely to see things like #1, they pre-shrink the images down so you can download them very fast, then click each image if you want to see it entirely.
With some clever picture windowing you might be able to impliment #2 into mIRC, but without some complex script resizing the images for thumbnails from the sending person, you'll never get #1..
