If scanning photographs – save them as EPS or TIFF files, this will preserve the colour and the clarity of images.  JPEG’s are fine but because they utilise variable compression technology it is preferable to save them at the maximum quality. 

When scanning consider the final size the image will be used at, always scan photographs at a minimum of 300dpi at final size and save as CMYK, avoid low resolution images from the internet.  However if you are working with images which are going to appear on a large-format, a minimum resolution of around 80dpi will be fine.  Don’t enlarge scanned images in your drawing/vector software, it is always best to use an image-editing application such as Photoshop for this task.