When to use IMG vs. CSS background-image?

Proper uses of IMG

1. Use IMG if you intend to have
   people print your page and you want the image to be included by default.
   —JayTee
2. Use IMG (with alt text) when the image has an important semantic meaning, such as a warning icon. This ensures that the meaning of the image can be communicated in all user-agents, including screen readers.

Pragmatic uses of IMG

1. Use IMG plus alt attribute if the image
   is part of the content such as a logo or diagram or person (real person, not stock photo people).
   —sanchothefat
2. Use IMG if you rely on browser scaling to render an image in proportion to text size.
3. Use IMG
   for multiple overlay images in IE6.
4. Use IMG with a z-index in order
   to stretch a background image to fill its entire window.
   Note, this is no longer true with CSS3 background-size; see #6 below.
5. Using img instead of background-image can dramatically improve performance of animations over a background.

When to use CSS background-image

1. Use CSS background images if the
   image is not part of the content.
   —sanchothefat
2. Use CSS background images when
   doing image-replacement of text eg. paragraphs/headers.
   —sanchothefat
3. Use background-image if you intend to have
   people print your page and you do not want the image to be included by default.
   —JayTee
4. Use background-image if you need to improve download times, as
   with CSS sprites.
5. Use background-image if you need for only a portion of the image to be visible, as with CSS sprites.
6. Use background-image with background-size:cover in order to stretch a background image to fill its entire window.