Image Quality
A digital scan is only as good as the source image from which it is taken, so you should always try to find the highest-quality illustrations. This often means the largest illustrations, because those usually have the most detail and will look best when zooming in. You should not be surprised when a scan from a small illustration looks bad on the big screen.
Image Capture: Digital Camera or Scanner?​
This is largely a question of personal preference. When using comparable equipment, there should be no real difference in quality between a camera and a flatbed scanner. Digital cameras can be faster, but they can also be more expensive.
File Format: TIFF or JPEG?
TIFF and JPEG are the two most commonly used file formats for still images. TIFFs are larger, more stable files that are ideal for maintaining a primary copy of your images. JPEGs are smaller, more nimble files that are suitable for everyday use in presentations and on websites. TIFFs are lossless, meaning that you can make changes to them without any degradation in the quality of the image. JPEGs are lossy, so every time you resave an image in this format it degrades slightly.
The VRC recommends that you save your images as uncompressed TIFFs and then make compressed JPEG copies of them for daily use. Scanning only JPEGs can be faster and easier in the short term, but it may end up causing you problems sometime down the road. TIFFs will take up considerably more memory than JPEGs (so you may need to invest in an external hard drive or cloud storage to hold them), but if done correctly, you should not have to rescan the same images in the future. However, you will still need both formats, because TIFFs are generally too large and slow to work in a PowerPoint presentation or on a web page. An imaging application like Photoshop can make derivative JPEGs from your TIFFs automatically.
Image Size: Pixel Dimensions
Usually, the most important factor in the quality of a digital image is the number of pixels it contains, because that determines what level of detail it can achieve. This is most commonly expressed in the image's pixel dimensions (width by height) or, particularly in digital cameras, megapixels (width times height divided by 1,000,000).
The VRC recommends producing ​TIFFs that are 3,000-4,000 pixels on the long dimension and making derivative JPEGs that are at least 1,920 pixels on the long dimension. You can make even larger TIFFs, but unless you are working with original objects, you probably will not be able to capture very much more detail that way. Most of our classroom projectors now display 1,920 ​​pixels (width) by 1,200 pixels (height), so there is no real benefit to using JPEGs any larger than that in your presentations.
Resolution (DPI/PPI)
DPI stands for Dots Per Inch, which is really a printing term. The proper term to describe images viewed on a computer screen is actually PPI, or Pixels Per Inch, but the two terms have come to be used interchangeably. As a ratio, PPI or DPI is used to translate an image from analog to digital format and back. The PPI of an image is less important than its pixel dimensions in determining overall image quality, so you may want to adjust the PPI to achieve your target pixel dimensions. For example, scanning a 10-inch photo on a flatbed scanner at 300 PPI will produce a 3,000-pixel image, while you would have to scan a 5-inch photo at 600 PPI to attain the same 3,000-pixel dimension. Note also that PPI is irrelevant when using a digital camera, because the size of the camera's sensor (in pixel dimensions/megapixels) determines how large the image will be.
Further Information on Imaging
