Check "Settings", "Reading", "For each article in a feed, show", did you select "Summary"? If you selected "Summary", your post images will not be included, and only part of your post text will show up in your rss feed.
Also, depends on what WordPress theme you are using, some might display your full blog post in a big chunk of text, no images, no paragraphs, no format at all. Anyway, try select "Full text" in "For each article in a feed, show", and see if that solves the problem.
Secondly, if you are using Feedburner to manage your rss feed with the default setting, your rss feed might show up without any formatting. Sign in your Feedburner account, go to "Optimize", enable "BrowserFriendly" and "SmartFeed".
BrowserFriendly would apply clean visual formatting to the raw feed XML for much improved readability in all modern web browsers, and SmartFeed would publish and translate your feed into a format (RSS or Atom) compatible with your visitors' feed readers.
