iMessage slow sending pictures.

When I use Whatsapp or WeChat or Skype or other IM Apps to send photos to my friends, photos will be sent very quickly.

Only when I use iMessage will it take me nearly 3or5 minutes to send the same photo to the same guy.

iMessage and Whatsapp or Skype are on the same network, and it won't make a difference between Wi-Fi or 3G.

Only pictures. Text will be fast. Is there anything wrong with my setting or with the network? Or is it because of the serves?

The problem occured many times on my iPhone 4 / 5c / MBP 2011 LATE / Mac mini 2011 Mid. So it seems that nothing wrong with my setting...

I'm in China, sorry for my poor English. Need help~

PS: And it seems that we chinese people also have problems on the App Store(both iOS's and OS X's) that it's also very slow.