It generally happens due to visual editor misplacing HTML when you switch between visual editor and HTML. The newer versions of Appdev fix this issue by creating a shortcode for the iPhone Slider and shortcode is lot easier to use as well. Pls have a look at the end of this page for actual shortcode –
The image size recommended is 323px by 553px (or higher but maintaining the same aspect ratio).
if you cannot upgrade right now, pls leave as a private message here with the site URL and a temporary login to the WordPress dashboard and I can get it fixed. You may also email me the same (see bottom right box in my profile page). Thanks