When to use IMG vs. CSS background-image?

Answer 1

Above answers considers only Design aspect . I am listing it in SEO aspects.

When to use

1. When Your Image need to be indexed by search engine
2. If it has relation to content not to design.
3. If your image is not too small ( not iconic images ).
4. Images where you can add alt and title attribute.
5. Images from a webpage which you want to print using print media css

When to use CSS background-image

1. Images Purely Used to Design.
2. No Relation With Content.
3. Small Images which we can play with CSS3.
4. Repeating Images ( In blog author icon , date icon will be repeated for each article etc.,).

As i will use them based on these reasons. These are Good practices of Search Engine Optimization of Images.