The most awesome feature of facebook chat is that we can chat with our ; or our friends profile pics..!! Am sure that almost 90% of facebook users doesn't even know about this feature.Well ,Here is a simple method to do it. Am sure your friends will get surprised when you do this first time.
Step 1 : First step is to collect the UID or Username Of the friend
for example : https://www.facebook.com/profile.php?id=01940005835
Here 01940005835 is the UID
or in the case of https://www.facebook.com/johndthottam
Here johndthottam is the username
step 2 : copy that UID or Username in the following format to chat box
[[UID or username]]
After that hit enter. Am sure you Will Love It . :-)