I recently made a single serving site for web designers that just had images of the finger cursor as a transparent PNG available to use in peoples Photoshop mockups. The only images on the site were two 20×20 pixel PNGs. When I posted the link to Facebook no images came up as thumbnails next to the link. I’m guess this was because they were too small (either in dimensions or file size, I’m not sure) to be grabbed by the Facebook image scraping script. I tried uploading a large image of a finger cursor instead, but still no good. After a little bit of googlin’ I discovered that Facebook only updates their images for a URL every 24 hours, but you can force an update by entering your URL here:

http://developers.facebook.com/tools/debug

It will also tell you what images it’s finding and give you other helpful information for any URL you are planning on posting to Facebook. Quite handy indeed.

You can also specify through code which image you want Facebook to use for a thumbnail by adding this to the head area of your html:

<meta property="og:image" content="whatever_image.png" />