You should have guessed by now....
Replace <img ....> by <img ... />. Within XHTML you need an ending tag, like <p></p>. Since some tags don't have an ending (like <BR> and <IMG>), you can usually solve it by replacing > by />. Another 'famous' one is <HR>.
I also found that epubcheck doesn't like it when you use upper and lowercase mixed. So for example <P>...</p>. So make sure you either do only upper or lowercase to avoid confusion.