Here’s an update. I completed graphics for basic alien types. It’s black&white here, but I color those differently in the game:

Also created some new bosses:

You can try it:
http://bigosaur.com/blog/droneinvaders.jar
None of the options in the main menu work, so don’t bother to try. Just click in the middle of the screen to play. All the levels are playable and there are new bosses each 10 levels. Most of the powerups (triple shot, auto-reload, etc.) work. You cannot die, so it’s nice for testing 
To raise the shield, click the shield icon in top-left corner. To detonate the atomic bomb, click the icon in top-right corner. This icon only shows up when you have bombs.