I don't have a ton of experience with RSS, but I think you may be seeing some formatting that was added to the post contents when it was written. On the other hand, formatting that relies upon your CSS stylesheet will not show through. That's why you don't see any margins around pictures that are included with my RSS.
In terms of pictures, I think that it depends on how you link to images. I recently learned that the "preferred" method of doing so is to start your SRC with a "/" which says "start at the root of my site" and then append your directories as appropriate. So for instance, if I wanted to link to a file called "fat_cat.gif" that resides in my images/artwork directory, I would create my link like this:
Images that I like that way generally come through (assuming that the person doing the reading isn't behind a corporate firewall that prevents the delivery of images from my domain).