1. Images such as pen drawings and line illustrations, which do not contain shades of grey or colour, should be supplied as 1-bit images, commonly referred to as 'line art' or 'bitmap'. The resolution should be in the range of 600-1200 dpi (dots per inch). These images may appear slightly jagged when viewed on the computer screen, because the screen is very low resolution compared with the image. Printing these images to your laser printer will demonstrate their quality.
2. Images such as photographs which contain shades of grey and colours, should be supplied as 300 dpi. There are many kinds of image formats that can be used, JPEG and TIFF being most common. When using JPEG, always use the highest quality settings available. If file size is not an issue, then TIFF is preferred for its ability to retain full image quality. In general, there are very few limitations on the formats that can be supplied, but it is extremely important to note the following with regards to the physical size of the image.
One important thing to note regarding image resolution is that size and resolution are inter-related. An increase in physcial size will result in a decrease in resolution. The opposite is also true. In practice, the final size of an image---the size it will be printed at---will determine the resolution of the image. For instance, if you have an image that is 3cm x 3cm at 300dpi, and that image is to be printed at 6cm x 6cm, the 'effective' resolution of the image is actually 150dpi, which is too low for quality printing. Output the image to a printer to examine its size to make sure it is large enough at 300dpi. High-end printing on high-grade paper may require a resolution higher than 300dpi. Some low-quality paper will require a resolution of 240dpi, and this should be considered an absolute minimum.
Images downloaded from the Internet are not usable. While the image may look fine on the screen, the effective resolution of the image is around one quarter (25%) of the size it appears on the computer screen. Unless the image has been specifically sized for printing, which is extremely rare, images downloaded from websites are not usable.