You would need to regenerate your thumbnails, to fit the new theme settings appropriately. The images, when uploaded previously while using the old theme, were sized specifically to that theme, then when you changed themes, which may have a different size that it uses for those same placements, has not had the chance to run their sizing functions appropriately and accordingly.
There are a few plugins that can handle the regeneration of images, which will basically resize all your images according to the ‘new’ settings of the theme or plugins that are using image sizing functions.
Couple Reference Points:
http://codex.wordpress.org/Function_Reference/add_image_size
and a plugin reference:
https://wordpress.org/plugins/regenerate-thumbnails/