If you have a tenant with a lot of users or a SaaS platform that works with millions of users accounts in Microsoft, it is extremely painfull and time consuming to pull/sync profile images from Microsoft Graph. Doing live queries against Graph for photos are also causing additional strain on the Graph infrastructure and end users suffer the performance penalty of a service having to query another service first. So it is ideal to sync images in some cases to local systems for performance gains.

Based on testing that I did I calculated that fetching photos on 300'000 users can take up to 4 hours. Reason for this is because you first have to fetch all the members and then fire a separate query per user to https://graph.microsoft.com/v1.0/users/{userid}/photo/$value to getch the photos, while staying within the throttling limits of Graph.
The other problem is also that the query to fetch the photo is blind as it may not return a photo as the user does not have a photo. But since I do not know if the user actually has a pohoto, I am forced to fire them blindly and then based on the response I will know if the user has a photo or not.

So, to resolve this two things would be required:
a) The user object needs a photo property populated with a last-updated date or etag.
This will tell me firstly, the user actually has a photo, stopping the need for a blind query against $value, and then secondly, is the photo the same as the previous time queried based on the photo property value.

b) Delta queries and subscriptions need to adhere to the photo property change of the user.
This will allow me to setup a webhook or timer job that will only query and work with user records that actually got changed, reducing the need for me to pull all user accounts from graph and then in turn reduce the amount of calls required.