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:

1. Passing for example bgurl=http://hendry.iki.fi/bg.png at the syslinux boot menu
2. 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:

https://github.com/Webconverger/webc/blob/master/home/webc/webc.sh#L138