I encountered this very issue myself and tried everything in the book to correct this bug/error..
This is VERY important, I was getting the error due to using images in a gallery, NOT from the images placed into a page!
This was the error received when performing the page analysis on pages that contain a gallery;
"No images appear in this page, consider adding some as appropriate."
Tried ALT tags, title tags to NO avail. Edited the theme files and even tried modding Yoast's core files..
Eventually I found a very simple fix, it's dirty ok but works.
Insert your IMG ALT text AFTER the shortcode but BEFORE the closing tags, like so;
[lightbox group="photos1" info="true" link="http://mysite/piccy1.jpg"][image src="http://mysite/piccy1.jpg"]<img alt="piccy1.jpg">[/image][/lightbox]
Note the img alt before the closing tags
Again it's a dirty workaround but works!