WordPress Gallery

This is the default WordPress Gallery that comes standard with every installation of WordPress and is not dependent on our theme. The size of the thumbnails is set in WordPress Media settings and is not controlled by the theme. For different size thumbnails recreate them using the Regenerate Thumbnails plugin.

TIP: if you make the thumbnails large enough you can scale down by changing the number of columns to display, and not lose image quality.

There are 2 plugins that you could use instead of the above.
NextGen Gallery and FooGallery. Both have options that you can easily apply.