floats not working

I’m a total newbie at this and I am stumped trying to get floats to work. I have searched through the forums and the codex and still no luck.

Briefly: I am trying to build a gallery of thumbnail images with captions. (I tried WPG2 and even that doesn’t align the images correctly; the first two columns are OK but the third image in each row is slightly lower than the other two.) I found what looks like the perfect solution in the Floatutorial (https://css.maxdesign.com.au/floatutorial/tutorial0405.htm). Following the appropriate section of that tutorial, I added the classes .thumbnail and .clearboth to the CSS, then added the recommended text to the HTML, starting with a sample of six images to be laid out in two rows of three.

Unfortunately, I got one long column of six images. The “clearboth” property seems to be working correctly, as there is a larger space between images #3 and #4.

The theme I am using is Painter, in case that makes a difference. The page I’m working on is not published; I’ve been saving drafts and previewing. However, I had a similar problem with my homepage (https://www.visiblelight.net) trying to get WP_cycle to float; I came up with a workaround for that (one that’s too klugy to use for an entire image gallery) but maybe you can tell something from the source code on that page that would shed some light on this problem.

Thanks in advance for any help you can offer!