A graphic image saved in WP's WPG format is a vector image.
Vector images have their drawbacks.
They're not Web-compatible for one thing. This is where bitmaps come in. The real advantage to bitmaps is that you can alter the image down to the pixel level. Presentations provides a converter for vector images, and in 3 or 4 clicks, your image will be ready for bitmap editing. Presentations also has a bitmap editor, but you must first convert a .wpg image to a bitmap before you can alter the image.