Websites and web applications need to be compatible with the following web browsers (current and previous versions unless noted otherwise):
Content-related images must be part of the HTML, not in the CSS as a background image.
For content-related images, alternative text must be provided through the alt attribute of the <img> tag.
Tips on how to write effective alternative text (WebAIM) (link is external)
For decorative images (not important to the content), an empty alt attribute must be included. Doing so will cause screen readers to completely skip over the image. If the alt attribute is omitted instead, screen readers will read the filename, which could cause confusion to those using the screen reader.
For images used as links, the alternative text must describe the linked website or the action caused by clicking the link.
Default Aspect Ratio: 16:9
File formats: jpg, png, gif
Minimum DPI: 72