1. Go to images.google.com
2. Search with your search term ("DNA structure")
3. Click on Search Tools
4. Set "Usage Rights" to appropriate option
5. Enjoy the wide variety!

I really like figuresearch.askhermes.org, which has indexed over 5,000,000 full-text articles and can give relevant figures with full captions. Most licenses are CC BY (free to repurpose with attribution) and you can specify in the advanced search menu.

Tons of lovely detailed figures on DNA structure, though protein synthesis brings up more experimental figures and there's no real way to filter them out.

I just went to look for Pubmed Central's image search (introduced in 2012) and it seems to have been removed...

Wikipedia commons: https://commons.wikimedia.org/wiki/Main_Page

https://commons.wikimedia.org/wiki/File:Protein_synthesis.svg

I wanted to add a few of the other sites I use.

NHGRI Digital Media Database: http://www.genome.gov/dmd/

NIH photos and illustrations: http://www.nih.gov/about/nihphotos.htm

NSF multimedia gallery: http://www.nsf.gov/news/mmg/

CDC--kind of an odd and often creepy collection of disease stuff: http://phil.cdc.gov/phil/home.asp

One point though: although many of these images are government made and free to reuse, not all of them are. So still check.

Often I took figures from Scitable. I found many interesting and scientifically correct figures. Of course, you have to report the source of the figure, because I guess that the Nature publishing group have a copyright on them.