ayoube Their images would likely be sent from their CDNs, if you block the entire domain like reddit.com without www. It should block anything that's let's say xyz.reddit.com etc.
If images still pass you can check your logs as they may be using some other CDN/Domain to deliver the images and you can block it that way.
So by blocking the entire domain and at worse looking at the logs you should be able to block their images quite easily.
I hope this helps.