[TYPO3-ttnews] Three images for tt_news

[Xatucans]

Hi everybody, I want to use 3 different images in tt_news, the first one is
a thumbnail for list mode, the 2nd is middle size one for single view and
the 3th is a big one for the popup window displayed when you click over the
single view image.

I have no problem with the 1st and the 2nd images, but I don´t know how to
display the 3th inside the popup. I write this code in my site's template TS
Setup:

plugin.tt_news.displayList.imageCount = 1
plugin.tt_news.displaySingle.imageCount = 1
plugin.tt_news.firstImageIsPreview =1

when I click over the single view image the same image appears in the popup
(the 2nd one). Can anybody tell me what must I do to display the 3th? Thanks