In 'Thumbs' mode, the gallery module will display the thumbnail version of your images. In this example, our thumbnails are set to 150 x 150px in the media library image settings. If you change this to say, 200 x 200px, BB will display that size of image in the gallery module.
In this example, we've used the 'Collage' mode and chosen 'small' images with a 5px spacing. The Gallery module doesn't set the positioning of the photos automatically so you may need to adjust the location manually by dragging the images in the order that best suits the layout.
In this example, we've used a collage of 'medium' images with a 5px spacing. The order of images needs to be adjusted again to suit the layout.
In this example, we've used a collage of 'large' images which forces the layout to display each one in order. We've also selected the caption to show on hover.