Select the image you want to convert. From the Objects menu, click on Rastorize. A window appears. Select your color model and resolution. Click to put a check mark if you want to anti-alias or to make a mask.
Resolution refers to pixels per inch.
The higher the resolution, the better the quality, but the bigger the file size. Anti-aliasing smooths out jagged edges. Masking makes the background transparent. Once your vector image is converted to a bitmap, you can no longer apply vector tools and commands to it. Inc. How things get done.