Has anyone had any luck adding text or buttons to the image preview screen. When you click on the image and look at it, you have the option to change the size, and the little cart icon. I would like to add some text and maybe a button there.
What you refer to is the lightbox which shows you the photo as big without any thumbnails at the sides. Below the photo, you'll see the caption of the photo if there is a caption present.
You can add or edit a caption outside of lightbox view. If you're using SmugMug style (with the thumbnails on the left and the larger photo on the right), you'll be able to add/edit a thumbnail right below the large photo on the right via the add caption / edit option. If you don't see the option or use another style, you can use the Tools button > caption / keyword tool to edit the caption.
In captions, you can also use html code so you could not only add text, but also images or links. To prevent the html from getting filtered out, enclose your caption in and tags.
You can find additional information on captions on this help page: http://help.smugmug.com/customer/portal/articles/93290-photo-captions