Field in Custom Content Type Stripping out  tags
A custom field doesn't have the same attributes as the standard body area.

Another approach would be to add a second field for the image and separate the two types at the content entry stage. You can combine them using Views. You could hide the first field and then rewrite the second field to display them together. A possible problem here is that the image will appear at the beginning or end of the copy -- float the image to have the text wrap around it -- but you won't have an option to place anywhere else.