xChiyomiChan [2014-04-22 03:16:05 +0000 UTC]
I believe you can only put thumbnails in your descriptions or comments if you are a Premium Member. Once you are a Premium Member, you can either search for it after clicking +Add Media, or copy and paste the string of numbers listed under the "Details" section of a deviation (:thumb123456789: as an example). Hope this helps.
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