Please see the update and corrections below!
As you can imagine, I try to keep on top of the changes in the WordPress.com Gallery feature and it was a pleasant surprise to find that two new features have been recently added to it.
Shape: You can now choose to display your “thumbnails” as circles, squares or rectangles (mosaic tiles) rather than the default thumbnails, which until now have been either landscape or portrait according to your uploaded image. If you’ve inserted irregularly sized photos in your Gallery this will now even out the thumbnail display of those images. It does not affect how those images appear when viewed individually, either in the Image Carousel, the photo’s Attachment page or at full size.
According to the current Support doc information, it does seem that the new default for Gallery thumbnails will be “square”, so in order to set the shape of your thumbnails to either circle or rectangle (mosaic tile), you will have to edit the Gallery shortcode in the HTML/Text Editor as follows:
[ gallery type="circle" ] or [ gallery type="rectangle" ]
The first choice will cause your Gallery to display like this:
Mosaic: Again, to give your Gallery a more homogeneous display, you can also set all your Gallery photos to display as a Mosaic by checking the box in your Dashboard>Settings>Media>Image Gallery Carousel>Tiled Galleries> Display all your gallery pictures in a cool mosaic. Checking this box will make all your Gallery look like this.
Mousing over any image in the Mosaic will cause the Caption to display. While this will change the appearance of your Gallery thumbnails, it doesn’t change the Image Carousel display (at least not yet), nor does it appear that you can set which images will display larger than others in the mosaic.
NOTE: These are very recent changes and may not yet be set in stone, so if you see some wackiness in your Galleries, these changes may be the reason.
UPDATE and CORRECTION: Now that the official announcement has been made, it appears that “rectangle” is in fact the tiled mosaic, so you can control the display of Galleries on a post-by-post basis as long as you don’t check the “Tiled Galleries” box in your Media Settings. My post above has been corrected to reflect this and Gallery-A Demo Blog shares some of the caveats of using these new options.