This is a security feature. ;)
You can allow adding img tags on all user input fields by adding it to the allowed tags field in the admin interface.
1. /admin/site-map/
2. Look for link named "ACS Kernal" at the bottom of the page
3. Look for "AllowedTag" on that page (ACS Kernal Parameters).
4. Add IMG to the field
5. Try again.
Caveat: XSS attacks (see: http://en.wikipedia.org/wiki/Cross-site_scripting)