Not sure if you want to learn Smarty (the template language) but you can look for the excerpt.tpl or blog_item.tpl file under the template you are using and look for a line that has this:

thumbnail file=$page.associated_filesi|i].file_path type=$page.associated_files[i.file_type maxw=100 center=0

the center setting is either on: center=1
or off: center=0

The thumbnail function can also handle max width: maxw
and max height: maxh

To help you scale your images.

I do not use center but you can see my custom portfolio site that runs with Siteframe here:
http://www.cglusky.com

How's that for a plug.

i cant remember how i get it done..probably through trial and error but here is my site : http://www.lowcq.com/home/ if you wan i can send you zipped of my template dir and maybe the index.tpl