Problem: On IE8, NextGEN Gallery thumbnails are either missing completely or the thumbnails are all squeezed together.
WordPress theme: Twenty Eleven child theme.
Solution*: I added the following code to my child theme’s CSS:
These pixel amounts are specifically for my needs. Make sure the height and width is the same as in Gallery > Options > Thumbnail settings.
NextGEN Gallery, v1.9.3
Child Theme based on Twenty Eleven, v1.4
*Disclaimer: This may not be a full/working solution for all circumstances or WP themes.