Category view and "Displayed image subtitle"

[Alocin]

Hi everybody!
I've some questions concerning the Category view display.
I'm trying to understand what's the meaning of "Displayed image subtitle"...It doesn't show up!?

I'd like to show categories in this way:



Should I have to make a new template? What have I to modify?
I'd like too to have a different color box for each category (as shown in the image). Is it possible? or...should I "draw" in HTML the page displaying categories and link each image with the corresponding category?
Thanks in advance for your suggestions!