I remember this being easy with xsplash but I'm guessing after double checking Ubuntu no longer uses it?
I would take a look at the links below. It would seem Ubuntu uses a package/daemon called Plymouth. Plymouth has its own scripting language, aptly named Plymouth, for all of its flashy. The image is a simple PNG image. In the /lib/plymouth/themes/default.plymouth file there are settings.
This is where the service is ordered which image to load and which script to load. For flashy, I'm guessing you'll need to script it. I looked for a few seconds and found many, many guides.