I quite like this idea in principle. Here's a few thoughts (in no particular order):
The sample image you provided is perhaps a little on the large side: maybe reduce the dimensions by 25-50%.
Would there be any limit to the number of these you could use in a single node? My concern here is that threads could become very cluttered if this facility was abused.
Setting up some formal HTML class attribute names (e.g. <img class="imagemacro advicedog" ...) would be useful for those using CSS.
Having a minimum level is probably a good idea; although, maybe pick something lower than Friar (level 9). Perhaps choose a level that currently doesn't have any special privileges, e.g. Monk (level 7).
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