Hi pouder,
I don't quite see how "messenger photos" is related to Live Mesh?
Generally, increasing the size of thumbnails to match their original size usually causes high amounts of image distortion and loss of quality, basically because the computer needs to 'estimate' the colours of the new pixels of a higher resolution. Bi-cubic Smoother is a technique used to enlarge pictures while maintaining the best image quality, however in the end, no technique will give you the original image back.
Anyway, I suggest you post your query into a more relevant forum (such as Windows Live Messenger). Even so, WLM display picture thumbnails would still not be able to resized without the problems discussed above.
ShadowXVII | http://www.shadowsplace.net