A recently introduced feature is the ability to display a gallery as a slideshow. The images are displayed against a dark grey frame – see example in the wp.com blog announcement. That frame may not be to your liking, or may not fit the theme you’re using, but it isn’t directly customizable (yet?); so here’s a workaround:
Paste into the HTML post or page editor. Where I’ve written HEXF and HEXB, you write the hex codes for frame and border color respectively. For hex codes see “Colors” under Links 1 in my sidebar.
• If you want no frame at all and the background of your main column is white, use the hex code for white: fff.
• If the images are captioned, change the height and margin numbers to 470 and 495, respectively.
• In some themes the slideshow doesn’t behave as it should: you may need to specify an absolute width (for instance 600px) instead of 100%, or you may need to adjust the height and top margin numbers to completely cover the original frame.
• For galleries in general, see Jennifer’s reference post.