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://hendry.iki.fi/bg.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: