well you will need a plugin that is able to steal pics first, stuffplug is a good one, and there are others on this forum, go to
this thread to find some good ones. then you type in your convo "/config" and the last tab in the config window is the plug-ins tab. once you have installed the plug in, say for example stuffplug then you activate stuffplug and restart messenger. stuffplug automatically steals pics. to see which display pic's/custom emoticons's you have stolen type in your convo "/xcedpstealer"
hope this helps
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