Default value is >= 800px. So the grid doesn’t show correctly.
If you want use a small image you need to make this changes:
open file functions.php (folder fastgallery)
find this:
rnadd_image_size( ‘fg-normal’, 800 , 800 , true);
replace with:
rnadd_image_size( ‘fg-normal’, 300 , 300 , true);
PS: 300 is a small value images.