You can make use of IMG in case you are looking forward to enable people to print your page and the image has to be present there by default.
For images that have important meanings, you can go for IMG with alt text. A notice or warning icon are good examples of such images. Doing this will help communicate the meaning of the image to all user-agents, including screen readers.
In case you are dependent on browser scaling to render an image in proportion responsive to text size, IMG provides you the right fix.
You can also go for IMG if multiple overlay images are there in IE6.
Its a really wise idea to put IMG into use rather than background-image, as it paves the way to boost performance of animations over a background.
When to use CSS background-image
If the image is not part of the content, you can use images from background.
Pick images from background when while performing a task of image-replacement of text.
Another great benefit of using images from background is improvement in download times, as with CSS sprites
If you are trying to make the image visible, the best way is to use images from background. Dont forget to use background-image with background-size:cover if you want to stretch a background image to fill its entire window.