Just a warning concerning OpenOffice Impress.
I love OpenOffice, but it is only compatible to a point. By contrast, many of the PPT files you'll find on this site (cough, Daejeon's, cough) will be well beyond that limitation. Opening one and editing it in Impress will likely render it broken. Even something like Jennifer's Mario game may come out a bit sketchy, but still functional.
Furthermore, OpenOffice by default does not embed image files, so be very careful to either truck your images in a directory with the PPT itself into work or find the embed option somewhere in the software to ensure your lesson's target vocab does not get reduced to colours and shapes (specifically, red and cross; grey and square).