It's the distro-logo.png image file which is being displayed when the screensaver shows. If OpenViX is still the same as my ViX4E2PROJECT builds (and I imagine it is as my base code was originally a fork from OpenViX) then you can find this at /usr/share/enigma2/distro-logo.png, just FTP in to this location and overwrite the existing file with your new one.
You should be able to change the size of the image to more or less what you like providing the name and file type (.png) stay the same, but because it is being used as a screensaver and the background screen goes darker you will probably want to play around with a custom image to find what suits you best.
Not sure if you are aware but you can change the screensaver settings in terms of the time allowed before it kicks in or even turning it off altogether. In my ViX4E2 builds (and so again I assume the same in OpenViX) you can find this setting at Menu > Setup > System > User interface > Settings > Show screensaver.