The answer is here already. Work on it. Here is more background.

• Footer on last page
• Header/Footer Settings Recap
• Dealing with Fields in Microsoft Word
• IF Field
• AutoText Fields - Incorporating AutoText or other Building Blocks in Templates
• AutoText field

Graham's example depends on you storing your large image as a building block. Here is a variation.

The structure of an IF Field is: {IF expression1 operator expression2 "truetext" "falsetext"}
The field above would have your small image in one building block and your large one in a separate building block. This is somewhat easier than placing the actual images in the field, although you could do that.