Changing default “Send to editor” img tag output

Greetings,
I’ve added a few buttons to the editor (not the WYSIWYG) to make my life easier but I also want to apply a default style to images that I insert into my post via the upload/browse dialog under the editor field. Any idea as to where this tag is crafted – or a better method (css?) by which this can be achieved would be appreciated.

Thanks in advance!