Lightbox: fix empty black box at left upper corner of page

[cathyxz]

This is probably due to the ImageViewer's initialization timing, but on first load or swipe of each lightbox item that has an image, the lightbox currently shows a weird looking black empty box with white outline in the upper left corner. Fix the ImageViewer measure call or initialization process so that this doesn't happen.

Update:
This is a funny bug. It doesn't apply to Desktop Chrome, but it applies to Desktop Safari, and mobile Safari, Chrome, Firefox. Basically they apply a gray border to images that have the alt attribute but not the src attribute.