Instructors and publishers will ask you to submit "good" or "high"  quality images. But, how do we quantify and qualify image quality? Most discussions on image quality will also discuss visual literacy.

According to the Association of College & Research Libraries (ACRL), visual literacy is defined as "a set of abilities that enables an individual to effectively find, interpret, evaluate, use, and create images and visual media. Visual literacy skills equip a learner to understand and analyze the contextual, cultural, ethical, aesthetic, intellectual, and technical components involved in the production and use of visual materials. A visually literate individual is both a critical consumer of visual media and a competent contributor to a body of shared knowledge and culture."

To  learn more about visual literacy, visit the ACRL Visual Literacy Competency Standards for Higher Education.  

Standard Four explains that "the visually literate student evaluates the aesthetic and technical characteristics of images," with learning outcomes including being able to evaluate "the technical characteristics of images (e.g., resolution, size, clarity, file format)," as well as evaluating "the quality of image reproductions, based on indicators such as color accuracy, resolution, manipulation levels, and comparison to other reproductions."

When evaluating image quality, the first thing we often do is look at the image! When searching for images consider the following as you look:

• Clarity / Focus: Is the image blurry? Are details visible?
• Noise / Watermarks : Does the image look overmanipulated ("Photoshopped")? Is there anything blocking part of the image?
• Color: Is the color probable for the subject of the image? If it is a photograph, are the colors natural?

(Two images of Jean Baptiste Joseph Wicar's Virgil Reading the 'Aeneid' to Augustus, Octavia, and Livia, 1790 - 1793, oil on canvas, Art Institute of Chicago. Image on the left was scanned from deteriorated 35mm slide film.)

One way to determine image quality is with size--there are three types of measurements to think about when considering image quality. 

• Dimensions: The height and width of an image, measured in pixels, inches, or centimeters. Also referred to as "image size”
• File Size: The memory size of a digital image file, usually measured in kilobytes (kb) or megabytes (mb) 
• Resolution: The density or amount of detail in a digital image, often measured in ppi (pixels per inch)

How exactly does resolution work? DPI stands for dots per inch and refers to printing. In practice, DPI is used interchangeably with PPI, or pixels per inch. A dot, or pixel, is a single dot of color in an image. Digital images are made up of pixels. An image’s resolution is measured by how many of those tiny dots are crammed into a certain amount of space. Generally, the higher the resolution, the more smooth the image appears, and the less likely the human eye will notice individual dots. An image with high resolution has more dots per inch than an image with low resolution. 

Images with higher resolutions take up more memory than similar images with low resolution. Because digital images are made up of pixels, the more pixels you have, the bigger the file size. Because a 300dpi image has 300 dots in every vertical inch as well as every horizontal inch, doubling the resolution actually quadruples the amount of dots in the picture, and thus, quadruples the file’s memory size! 

When searching for images, the quality you need depends on your intended use. For example, a publisher might require a resolution of 300 ppi for printing, but for a PowerPoint presentation a resolution of 72 ppi may be fine to use. Similarly, for a PowerPoint you would ideally want a pixel width of at least 2000 px on the long side. (Note: While there is always a temptation to capture a screenshot of an image rather than to download a file, screen captures will reduce the image resolution and quality.)

(Detail showing pixels from a wall painting with Achilles on Skyros from the House of the Dioscuri in Pompeii, ca. 60 – 79 CE, now in the Museo Archeologico Nazionale, Naples, as printed in P. Herrmann and R. Herbig, Denkmäler der Malerei des Altertums. Munich: Bruckmann, 1904)

[Adapted and expanded from "Preparing Images for PowerPoint, the Web, and Publication," A University of Michigan Library Instructional Technology Workshop.]

Just like you might need a different digital image resolution for different purposes, the same is true for file formats.  

[Adapted and expanded from "Preparing Images for PowerPoint, the Web, and Publication," A University of Michigan Library Instructional Technology Workshop.]