In our office, we setup a new network with mirkrotik router CCR-1009-7G-1C S+. Connection is fine but facebook images are not appear. So, I reboot the router CCR-1009. The images are appears but sometime later, the problem happens again and again. I would like to know why and how to solve?

All images on the web site have an url. Try to resolve IP address from dns name of these images? If you can't do this, this means you or provider blocks these dns.

as per my tiny knowledge, It's probably due to one of following ...

• DNS issues (Most common one)
  
  MTU problem (seen this issue at a network where many wifi routers were installed and the new TENDA brand was giving such issues)
  
  Packet is blocked or lost at any point / HOPS (less chances)

for #1 , make a rule to forcefully route all DNS requests to external DNS server. Example below ...
First try dns forcing rule, it may solve the issue