Prior to the Delve user profile page, the Share Point user profile page referenced images stored in Share Point rather than Exchange. 500Kb Provided manually by users or a bulk import can be scripted if source photos can be located and named appropriately.
If not provided, Exchange will reference the AAD thumbnail Photo in some instances but only if the thumbnail Photo is less than 10Kb.
Small is 48 x 48, Medium is 72 x 72, Large changes depending on the source image but is always square.
I have seen as small as 120 x 120 and as large as 300 x 300.
Please make sure you have a profile picture saved to your computer.