I noticed that when you set the Site Url of your site differently from the WordPress Url your plugin doesn't load the image previews correctly anymore in the administration (the site works smoothly).
I analysed the code, the images aren't loaded because the path is wrong, looks like your plugin use the site url instead of the wordpress url to generate it.
Say that I have my Site url set to http://www.mysite.com and my WordPress url to http://www.mysite.com/wp. When I load an image in a custom field, after I've saved the article the image preview will disappear because the path will be
which is where the image actually is.
Thank you for the excellent plugin, I hope this can help you making it even better.