Code:
<if condition="$post['fieldX']"><img src="path/to/image" /></if>
? If you want their custom option to appear also:
Code:
<if condition="$post['fieldX']">$post[fieldX]<br /><img src="path/to/image" /></if>
Put that in the template postbit/postbit legacy depending on what you use. Replace X with the field number (can be found in profile fields in ACP). Remember, in the actual code, $post[fieldX] has no '' around it.