Library Guides: Visual Literacy: Evaluate images

Technical aspects

File Formats

There are many different file formats for images. Some work better for particular purposes. For instance, JPG, PNG, GIF work well on the web, while TIFF files are best for high-quality prints.

Different formats have different levels of compression, varying file sizes and different capabilities, such as the ability to save transparency.

Resolution

Resolution refers to the quality and clarity of an image. In digital images, this is measured by pixels per inch (PPI), that is how many individual pixels are displayed per inch of an image. For printed images the measurement is dots per inch (DPI).

If the resolution of an image is very low (a low PPI or DPI), it can become unfocussed and individual pixels can become visible.

Raster & Vector Images

Many of the images you find on the web are raster images. Raster images are made of pixels and are resolution dependent. File formats include JPG, PNG, GIF & TIFF.

Vector images are comprised of lines not pixels. This means you can increase the image to any size without loss of resolution and with no increase in file size. Examples of vector images include most computer fonts and Microsoft clipart and graphs. Vector images are excellent for logos and text. Common vector file formats include EPS & SVG.

For more technical info on the University of Michigan's All About Images libguide.

Evaluate images

Any image you use for study, research, or for presentations need to be critically evaluated like any information resource.

The below questions will help you to
EVALUATE

EVALUATE
CONTENT	Take your time and examine the image. Describe what you see and what you think the image is about. Imagine trying to explain the image to someone who can’t see it. How are visual elements such as colour, shape, layout used in this image? Does the design of the image influence the message conveyed? Are their multiple interpretations?
CONTEXT	What information is provided (i.e. description, captions)? Does is alter the way you see the image? Does the text provide factual information or persuasive information that influence what and how you see?
SOURCE	Who owns the image? What are the origins of the image (creator, date)? Is the source you found the image trustworthy? Has the image been used in other resources? Has it been used in a different context to convey meaning? Are there copyright restrictions? Can I use this image?
IMAGE QUALITY	How good is the image resolution? Is it clear or grainy? Has it been changed in any way? Colour? Has it been cropped? Is the image in a usable file format?

Adapted from University of California Irvine, Visual Literacy: Evaluate Images, University of Washington Tacoma, Images: Evaluating Images

Metadata

METADATA = DATA ABOUT DATA

In the case of images, metadata refers to the information about the image. The amount and quality of image metadata is not consistent or standardised, especially on the web. Metadata can also be modified so it is important to review all available information critically.

There are different types of image metadata:

Descriptive metadata: can include the name of the creator, captions, comments, tags. It's usually this information that makes an image search more or less effective.
Administrative metadata: usage rights, copyright statements
Technical metadata: often auto generated by the camera

Books by Gael Varoquaux / licensed under CC BY 2.0.
Rouse, M 2015, What is image metadata? Viewed 24 March 2021 from https://whatis.techtarget.com/definition/image-metadata