Post thumbnail

Hi anyone

Can anybody tell me how to style og set the post thumbnail in a way that the post thumbnail only display a small thumbnail photo say 100×100 px – of the original post photo lets say 800×900 px.

Right now the post thumbnail show small thumbnail photo AND original large phote which is insert in in the textpart of the post.

I run Athualpha theme 3.4.9

Please help me out here – I am about to give up.
cwjorgensen