Hi,

It seems that it take some time to load the image since of the high size of the image and it might happen that the image is not loaded before size of the image can be calculated, therefore the background of the image is not displayed correctly.

Could you please try to increase Load delay time in “Load delay” property of your lightbox web part or control?

Load delay time (in milliseconds). If you are using automatic resizing, this value indicates how long will be lightbox wait for taking element size. If you have problem with displaying of lightbox content, try use upper value.

Please also try to store files in File system (http://devnet.kentico.com/docs/devguide/where_the_files_are_stored.htm) for better performance and also please use Generate thumbnails option:

http://devnet.kentico.com/docs/devguide/files_related_settings.htm

I hope this will help you.

Best regrads,
Miroslav Remias.