Images for the web or to be used in PowerPoint do not generally need to be that large. Most monitors will have a resolution of around 100-150 pixels per inch. Thus an image 1500×1500 will appear on the screen as between 15″x15″ and 10″x10″.” Extremely high resolution images are only particularly needed if displaying on a very high resolution computer or for printing.
In terms of format, the choices for the web are PNG, JPEG, and GIF and generally you will be much better off using either PNG or JPEG for all but the simplest of images. These formats, along with a number of others (such as TIFF), are also all supported in Microsoft’s PowerPoint presentations package.