WordPress theme issue !!

Hello there, I had a question about the lifestyle 4.0 theme, I am currently using it with wordpress 2.9.1 and can not seem to figure out how add thumbnail images to my post and page. I have tried using the custom field entry “thumb” however that simply just repeated the title of the post rather than placing a thumbnail image. I was hoping you could shed some light on this matter.

Thanks in advance.