The images you purchase may be opened and manipulated in any program which will handle images. If you have Photoshop, Photoshop Elements, PaintShop Pro here is a video that will help with changing the color and size of your digital stamps (you might want to turn down your speaker volume) She demonstrates in Photoshop, but it will be similar in the other graphics programs:

Here is another great video that demonstrates how to resize and flip images using Microsoft Word: