Albert with his Chalk

Received another postcard in the post today.  What does the circle represent behind him, perhaps the size of his brain's aura ;)  Looks like a dark form embedded in the chalk board itself looking out at him, creepy.

Found on the reverse side:

PSgoKGEvYikqYykrZCkqZQoKeD0xLHg9Myx5PTQseT03LHo9OA==

I know what this code is, however when translated it made no sense to me which is why I've left it in original form.  Maybe it means something to someone.