Types of images
Let’s start with a look at the two general types of image formats you’ll come across.
Bitmap (also called raster) images
When the smiley face in the top left corner is enlarged, individual pixels appear as squares.
This is the most common image type. Digital photos are bitmap images. Think of a bitmap as a grid of tiny dots with each dot mapping to a part of the image it represents. These dots are commonly referred to as pixels (px). Because the grid is fixed, it doesn’t expand and contract as the size of the screen changes so the image can fill a small screen but be lost on a large screen. In an ebook, it also means that the size of the reflowable text can change but the bitmap image remains the same size.
In practice, bitmap images are often scaled up or down but they lose quality: resizing basically just increases or decreases the size of pixels so the images appear fuzzy and edges are jagged. Special techniques can be used to resample bitmaps to improve image quality, processes known as downsampling and upsampling to reduce or increase image size. It is used by many e-readers and e-reading apps when the images they encounter were produced for a different screen size.
For ebooks, the most common bitmap file type is JPG (jpeg). It’s supported in all ebooks. Other common types are 
BMP (bitmap), TIFF (tagged image format), PNG (pronounced ‘ping’), and GIF.
Vector images
These do not use a fixed grid of tiny dots. Instead, they are built up from geometric shapes such as points, curves, and lines using mathematical functions (‘vectors’) to draw the images on the screen. The advantage over bitmap images is that they can be scaled without loss of quality.
A vector format called SVG (Scaleable Vector Graphics) will become important to ebooks in the future, providing the image equivalent of reflowable text. SVG is part of both EPUB3 and Kindle KF8 ebook formats.
Right now, device support for SVG is very limited so ebooks seldom use vector images.
How screen sizes affect images
As well as being measured in inches or centimeters, screens are measured in pixels which define their resolution.
Here are some screen sizes for ebook reading devices, including older devices. High resolution screens have been available for quite a while now so the ‘typical’ resolution to design for is getting higher. Screen sizes are usually quoted as width x height:
- 600 x 800 px for an original Amazon Kindle
- 1024 x 600 px for the original Kindle Fire 7″
- 1920 x 1200 pix for a Kindle Fire HD 8.9″
- 1448 × 1072 px Kindle Paperwhite
- 1080 x 1920 px for an iPhone 6 Plus and later screens
- 1080 x 2220 px for a Samsung Galaxy 6 and later screens
- 1024 x 768 px for an iPad 2 screen
- 2048 x 1536 px for the iPad Air screen (retina display)
- 2732 x 2048 px for iPad Pro 12.9″
The number of pixels doesn’t necessarily relate to the physical dimensions of a screen. So, for example, 1024 pixels can span the width of a TV screen, a computer monitor, or an iPad screen. This is achieved by making the dots (pixels), or the gaps between them, larger or smaller. This leads to another important characteristic you need to understand when you are creating images that will look good on various e-reader screens: dot (pixel) density.
The density of dots is usually expressed as dots per inch (DPI). Although it might seem that the higher the dots per inch, the better an image will look, this isn’t always the case. For instance, optimal viewing on the web or on many ebook readers, such as the Kindle, is 72 DPI.
In practice, this means that images should be 72 DPI to work well on a wide range of screens and the web. However, higher DPI (150–300 dpi) are often used now for ebooks as high screen resolutions such as Apple’s retina displays become the norm. This is pretty much universal now for cover images.
Resources
Find out more about this topic on our Digital Publishing 101 useful resources site.
