[leschek]

I'm trying to update Czech translation and here are a few strings I need help with:

1. In CRE documents, an alternative status bar can be displayed at the top of the screen, with or without the regular bottom status bar.
   
   Enabling this alt status bar, per document or by default, can be done in the bottom menu.
   
   The alternative status bar can be configured here.

   Here: https://github.com/koreader/koreader...tener.lua#L173
   
   I'm not sure what are CRE documents?
   

2. "%1"
   is not a valid image file!
   A valid fallback image is required in Cover-Image.

   Here:
   https://github.com/koreader/koreader.../main.lua#L100
   
   In this case I don't know what is Cover-Image? I didn't find it in any other language string. Is it name of some folder in KOReader's installation?