(Q) How can I use my digital stamps after I've downloaded them:
(A) Open a editing program such as Microsoft Word, Adobe Photoshop or Pages, and insert picture.

(Q) Which do I use the JPG or PNG version?
(A) You can use either, a JPG comes with a bounding box and works well as a single image. A PNG come with no bounding box, and a white fill for over lapping, and works well for staging and creating scenes.