Need to set defauts for image placement in editor.

Well, as much as I hate to do it, I have a need to alter the TinyMCE defaults on an installation ... specifically the image alignment defaults when an image is placed in a content editor.  The client cannot seem to grasp the concept of left aligning the images and giving a 10 pixel horizontal margin so that the text wraps even though I have showed him countless times over the years.  If I had known when I set up the section that he would want a photo and some text in this field, I could have made separate fields for image and text and controlled the layout on the front end, but it's too late now, so I have gone through all the entries and fixed them and now want to default ALL subsequent images in content editors to left justify with 10px horizontal margin.  Where would I start looking for the code that I would have to alter?  I realize it will affect all editors for all sections, but that's OK since 99% of all need left justified anyway.