The free MEDLINE Images database is made up of over 3 million images from biomedical literature which has been deposited at the National Center for Biotechology Information (NCBI), and is maintained by the National Library of Medicine. You can connect to MEDLINE Images at this link.
When you are browsing an image, you should see a link to the article that it came from. Remember that if you are looking for medical articles instead, you should first try PubMed Central (which has full-text articles), and then try PubMed (which is a mix of links to full-text articles and citation only).
The above image was taken from: PGC-1 coactivators: inducible regulators of energy metabolism in health and disease; J Clin Invest. 2006 March 1; 116(3): 615–622. DOI: 10.1172/JCI27794.