Featured Image Thumbnail size on Twenty Ten Archive Page

At first, make sure your thumbnails are set to size 150×150 in “Settings/Media”.
If they are not, you may have to regenerate your thumbnails after setting the new size. This Plugin helps you here

In your template, be sure to call the ‘thumbnail’ size of the image:

the_post_thumbnail( 'thumbnail' );

you should be fine after that. If not, please write the line of code where you output the image.