Here’s a very brief introduction to the way splash screens work in Xfce 4.2.0 and above:

The whole splash thingy is a bit complex atm. On the first level, there are splash engines (currently mice, simple and balou); these are loadable modules. mice isn’t themable at all, everything’s hardcoded. simple is themeable from the configure dialog, and balou is themable using a theme file.

balou theming is pretty easy, its just a themerc file and a logo image file. Check the Default balou theme in the xfce4-session source to get an idea of the file format/content (it includes a few comments). To distribute a theme, create a directory layout like this:

Name
Name/balou
Name/balou/themerc
Name/balou/logo.png

where Name is the name of your theme. Then tar up the stuff like this:

$ tar czvf Name.tar.gz Name

Now users can simply install the theme by dragging the .tar.gz file from a file manager and dropping it to the balou configure screen in the settings manager.