I pushed an update to my Last.fm 3×3 Album Art Image Grid for WordPress plugin that makes it a lot nicer to use day-to-day. The main change is that you no longer have to paste your Last.fm username and API key into the shortcode every time you want to show a grid. Instead, you can set them once in your WordPress admin under Settings → Last.fm Grid, and then just drop the shortcode anywhere you want the grid to appear.
Behind the scenes I also cleaned up the code a bit: it now uses the WordPress HTTP API (instead of relying on cURL), adds caching so it’s not calling Last.fm on every page load, and generally does a better job of sanitizing/escaping output. If you’ve been using the plugin already, updating should be straightforward — just set your username and key once and you’re good. Note that the plugin does not auto-update, you will need to download the new version here.