make sure to select 'aligne left or align right' when you insert the images into the post or page.
Twenty Ten has the styles needed for wrapping, however, it also has a clear: both; on the html h tags which prevents the wrapping if you use the headlines in your post or page.
please post a link to your site, pointing to a post or page with the problem.
try to use a tool such as Firefox' add-on Firebug to further investigate the css.
http://getfirebug.com/
