I've set the 'show the download button' to True, but people can't download the pictures.
Thank you for reporting this problem. I have been able to reproduce it in a test gallery of my own and have logged a bug report with the developers. Hopefully it will be fixed in the next version but, in the meantime, the best alternative would be to use the Open Image Button instead of the Download Button (not quite the same functionality but the user can download the image from the Flickr page).
Sometimes the page won't load on smartphones or tablets (Android and iOS).
Please make sure that the gallery is being viewed over a wi-fi connection rather than 3G. If 3G is being used, content modification can break the gallery, resulting in a blank page. Please see this FAQ for details and a solution to the problem.
Why can't I view my gallery on a 3G mobile connection?
SimpleViewer Support Team