I've installed image pack. Now what?

That is, from the PA Menu, click on the Options button and select Install a New App, yada yada yada.

When it's done, open Open Office Writer (or Calc, doesn't matter)
From the menu choose Tools>Options.
In the Options, expand the section called OpenOffice.org, and select View.
Where it says Icon Size and Style, there's a dropdown that now contains the different styles in the image pack.

Hope that helps.
You have a point, the DL page here could use a LEETLE more explanation of the Image Pack.