Nope, you don't need to buy anything.
You only need to download and install a plugin for Messenger Plus!, called StuffPlug-NG:
It can be found here:
http://www.stuffplug.com/
To activate the install, you need a validation code, which can also be found on the site (or on direct link:
http://www.stuffplug.com/?act=generate)
detailed install instructions can be found here:
http://www.patanes-uanl.org/spng/index_en.htm
To have enable oversized display pictures, you need to go to the configuration panel of StuffPlug-NG and tick:
"enable over-sized display pictures".
After that you can add a new display picture and StuffPlug-NG will ask you if you want to keep the original (larger) size.
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