Hello,

Is it similar question as other topic about image as background in module calendar, or a bit different question ?

I'm not sure to understand well here what you whis to accomplish...
If you mean to force a custom color instead of category's color for event days in calendar, you have the dedicated options in the module calendar options

Regards,
Cyril

Ok i see!

So, if not in css file, it's normal, as it is a php request from categories database table.

For now, the only working way to do what you want, is first to be sure to have always only 1 event per day.
And if yes, then to create on your own custom theme pack... But this supposes you have php/html knowledge...

If you have the skill to create your own theme pack for iCagenda, with it's own name and code, so i will be able to tell you which change you could do to load the event image as day background.
Small tutorial video to start with to understand how iCagenda theme pack work (outdated version but the how-to is the same) :
icagenda.joomlic.com/docs/documentation/...our-first-theme-pack

Best regards,
Cyril