Need help in build plugin

[HaCaoChien]

Hi,

I try to make my first Mod. I wanna make a mode that add watermark to all picture post in my forum. So, it's works when I post some images in test topic. But when I reply in that topic, the watermark was disappear. But if you edit those posts (̣just click Edit and then Save, nothing had been changed) the watermark was appear again

Attactment is my xml file. Pleas help me

Thanks so much