Sizing WP thumbnails

Hi, I look for solution in codex and forum but did’nt find.

When uploading a photo with basic WP function, WP is creating a 72 * 72 thumbnail, named name_photo.thumbnail.jpg. I look for a way to change the size of this thumbnail and did’nt find any.

Is there a solution to that (maybe I did’nt look for well on codex/support ?)

Thaks by advance.