thumbs

I really hope somebody can help me with this it’s driving me mad.
I have just bought a new theme the ,manual is not in english ,
im trying to build a amazon site using a autoposting plugin .when it post it displays the wrong image.
If I put in the custom field

Name:thumb Value:$thumb

it works fine but I have to go into every post to do it .my question is there a way to make it do that to all the new posts…thank you so much