I'm sure there is a 'correct' way of doing it in After Effects but you could always try this first in Premiere.

It's not in front of me so I hope it comes as a standard transition but what you want is a gradient wipe that takes a mono gradient to work from. Pixelan do a good one and you can create your own 'wipes'.

So create a gradient that runs in a circle and use that to 'transition' a still containing your circle into the picture.

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