Have you set it to allow em in the config of Stuffplug NG (in a convowindow: plus > plugins > stuffplug > configuration)?
From
Stuffplug NG FAQ page:
Features » Over-Sized emoticons
This feature allows you to have bigger custom emoticons than the default 19x19 pixels.
To use it simply enable the feature and add a new emoticon which is bigger than 19x19 pixels.
StuffPlug-NG will then ask you if you would like to keep the original size, just like over-sized display pictures.
Please note that people not having this feature enabled will only see the upper-left corner of your bigger emoticons.
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