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).