Thanks for your help! But I'm get stuck at the second step . I followed your instruction at the second step but it doesn't work, maybe because of my limited English ability! Do you mean:
- Create MACROBUTTON field via Ctrl-F9
- Paste "MACROBUTTON PicSize {IF{DOCPROPERTY PicSize}= Y "Pic1 Here" "Pic2 Here"}" between {}
- At the position of "Pic1 Here", I will go to Developer tab, and insert "a picture Content Control"; similar for the position of "Pic2 Here"
- Then select the field and press F9

I'm really thankful to you!