If you're using WordPress.com for a photo blog, or if you just post photos regularly, you'll definitely like the new gallery options. You can now add photo galleries to posts that go beyond the old and stale thumbnail list and have a more modern look.
"When adding a gallery to your post, you now have the option to select a layout style for your images. We’ve added support for Rectangular, Square, and Circular galleries," WordPress.com explained
You'll be familiar with the layouts as they're similar to what other sites are doing, apart from the Circular version perhaps.
The default tiled gallery is Rectangular in which thumbnails are adjusted to fit well together and not leave any open spaces. It's similar to the way image results are displayed on Google Search.
The Square layout uses same-sized thumbnails and crops photos if needs to make them fit. It's somewhat similar to what you'd see on Pinterest, without all the whitespace and text. Choose this one if you want a more orderly look.
Finally, Circular uses equal, rounded thumbnails for all the images. It's a unique look, but it may be a bit too eccentric for most uses. Still, it's nice to have it as an option.
"By default, galleries will continue to display using the standard thumbnail grid layout. To switch to one of the new layouts, head over to Settings –> Media in your blog’s dashboard, scroll down to 'Image Gallery Carousel,' and select the box next to 'Display all your gallery pictures in a cool mosaic'," WordPress.com wrote.
This option applies the selected gallery layout for the entire blog, so all of your previous galleries as well as future ones will be rearranged with this layout.
Alternatively, you can choose a gallery style for all of your posts. Leaving the "Display all your gallery pictures in a cool mosaic" unchecked means that you'll have to manually specify the gallery layout for all posts and that galleries will use the thumbnail grid by default.
To use a gallery style manually, you have to add the code [gallery type="rectangular"], replacing rectangular with "square," or "circle" if you want.