Twenty Ten gallery thumbnail issue

1. After I upgraded WP I decided to try out TT theme. I created gallery category and assigned some posts to it. They show up with thumbnails on the main index which is great, but I want to resize those thumbnails. After I changed the settings to something other than 150×150, nothing changed. I did notice that any new posts have thumbnails with my new desired size. How can I have them all be the new size?

2. I noticed that all the images I have been uploading up until now, have additional files with 150×150 and 300×166 tacked onto the names. So my blog has been crating thumbnails all along without me realizing. What I am confused about is why does TT automatically choose the 150×150 instead of the other one. I can’t find a setting that would define this.