Image Insert and Theming
I will assume you are referring to using the Insert module. 

Once you have inserted an item into a node field (most likely the body), it then becomes a simple string of text within that field and is processed as such. There is no longer anything linking it to the image field that you used to upload it, and no way to hook onto it and perform any programatic changes. The only "theming" that can be done to the image at this point is CSS changes.

You could potentially add optional styles for the images via your WYSIWYG's configuration or you could use the Image Resize Filter module to allow the node editor to change the size of the image and/or create a link to the full-size image.

If you could give some details about what exactly you are wanting to do to that images that have been added via the Insert module, there might be another solution someone can suggest.