JPEG file format only supports grayscale and True Color data types because it uses a lossy compression method. You can use Photo: Convert command to convert the image into grayscale or true color attribute, then you will be able to save it in JPEG format.

Before you scan, consider the purpose of this image since scanning in higher resolution gets better image quality but also larger file size. The following suggestions may be helpful for you to decide deciding:

• Scan for intended output device. For monitors, use 72 dpi (Windows large fonts)-96 dpi (Windows small fonts). For laser printers, use 300-600 dpi, and photo-quality 1200-2400 dpi.
• Scan at higher resolution (2x the original) when doing extensive image processing to work with as much information as possible.
• Scan at higher resolution when enlarging the image to scan.
• Scan at 1.5 to 2x the intended halftone screen's lines per inch when sending the file to a post production house to create halftones