http://en.support.wordpress.com/images/gallery/

Read it slowly (I did not, several times), above even has a link to below:

http://www.wpbeginner.com/plugins/how-to-add-gallery-carousel-in-wordpress-without-jetpack/

Followed some instructions, but in a hurry, so let’s try it:

Save Draft, Add Media, Select Pix, ??? :

Tiles… so that didn’t work, try: “Pencil” (edit the gallery), Nope, try the type=slideshow from text mode? Tiles.

OK, great, went back to settings, uncheck Carousel, check it, save, save, now if you click any image – pops up a manual slide show (have to right-arrow or click/tap edges of the screen to get the next image). Not exactly as advertised.

http://premium.wpmudev.org/forums/topic/why-dont-i-see-type-in-my-wordpress-gallery

* Remove carousel without jetpack, then install slimjetpack (all one word like that).

Mad. MADDER. MADDEST-EST!!

http://en.support.wordpress.com/images/gallery/
Back to here, slowly, uncheck the “show in cool mosaic.” UNCHECK.

http://premium.wpmudev.org/forums/topic/why-dont-i-see-type-in-my-wordpress-gallery
AND here:
http://wordpress.org/support/topic/missing-slideshow-feature-from-media-gallery
Back to here, Settings->Slim Jetpack->
Activate (1) Carousel (2) Tiled Galleries and (3) Shortcode Embeds

Now go back to Settings->Media->
…uncheck the “show in cool mosaic.” UNCHECK.
Save (at least twice because computers stink).

Now, at last Horacio!, flippy-floppy picture box above. Slideshow is a “Type” of Gallery when editing/creating.

Post-Facto: should have made this a post because it is showing up in my menu. Soon, copy it to a post and delete this page. Next-day Post-Facto: turned into a post, fixed some super-grammar and some layout issues. Deleted the page (and emptied the trash).