If I can ramble a bit more - forget the Anycast bit. If you run your own DNS server(s), you can just configure them to respond based on the geographic location of the requester. PowerDNS is pretty easy to set up for this. You could run your own DNS just for the image domain. You basically run PowerDNS authoritative server, set up your zones and the geoip stuff, then slap dnsdist in front of it to be publicly exposed. dnsdist has anti-DDoS features and loadbalancing in it, in case you need it down the road.

Since it’s just for static images, you can have a higher TTL so you don’t need to worry about distributing the DNS servers. (ie. the DNS lookup might not be super fast since it could go across the country, but it doesn’t matter since that lookup is only going to happen every TTL period on each client, which can be high.)