You can export from Keynote to PowerPoint.
But make sure to run it on Powerpoint before the actual presentation, it would be even better to test on a PC.
There are some features in each of Keynote, PowerPoint:Mac, PowerPoint PC that aern't available in the other.
You can also convert to Quicktime for or Flash.
You can search for clipart on the net like here:
FREE Clipart - 25,000 Images #1 Free Clip Art - Over 10,000 free clip art images! http://office.microsoft.com/clipart/