Boot artwork (the yellow tricolour progress bar) is not customisable.
From Webconverger 12 the background image on a white background seen between browser restarts can be customised by:
- Passing for example
bgurl=http://www.webconverger.com/img/logo.pngat the syslinux boot menu
- By specifying your background image URL in the customisation form and purchasing a subscription
Using option 2 is the most convenient, as this ensures you have you background set every boot.
Currently the background image URL is checked after every browser session, here is the relevant code and search for "bgurl":