Boot artwork is not customisable. Ideally users should never see Webconverger boot.
The background image on a white background (black for neon) seen between browser restarts can be customised by:
Currently the background image URL is checked after every browser session, here is the relevant code and search for "bgurl": https://github.com/Webconverger/webc/blob/master/home/webc/webc.sh
Turning off branding / throbber
The Webconverger brand mark can be turned off with
nobrand in your configuration.
Please see http://webconverger.org/blog/2014/New_logo/ for more information on the new logo.