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:

[css].ngg-gallery-thumbnail {
height: 110px;
width: 163px;
}[/css]

These pixel amounts are specifically for my needs. Make sure the height and width is the same as in Gallery > Options > Thumbnail settings.

Versions:
WordPress, v3.4.1
NextGEN Gallery, v1.9.3
Child Theme based on Twenty Eleven, v1.4

Resource:  [Plugin: NextGEN Gallery] Thumbnails not displaying properly in IE8

*Disclaimer: This may not be a full/working solution for all circumstances or WP themes.