Note that the <img> tag
must be inside a block element (a <p> or a <div>, typically). If it isn't, it is not valid XHTML and not a valid ePub... and the result is undefined.
Note also that the "alt" attribute should be some description of the image, to be used when the image cannot be displayed (think text-to-speech). If the image is simply decorative, leave it emply (alt=""), but do not write some cryptic meaningless stuff like "fig14.b_theNetDrill_wrong".