I noticed that with several browsers, images are not visible until page refresh.
This issue doesn't occurs on the demo site because the css is customized with fixed dimensions.
theme_image_style() should be used as it tries to get image dimensions, and rdf metadata is added.
The following patch resolves this issue by adding a image_tag property to $image, directly used by the nivo-slider template.
(I kept image and settings properties for backward compatibility)