If you saw the image in person at a museum or art gallery:
Last name, First name. Title of painting. 1900. type of medium (oil and canvas). Museum, City of Museum.
If you saw the image on a website, like a museum website or an artist's website:
Last name, First name. Title of art work. 1950. Museum, City. Name of Website. Web. 20 Aug. 2015.
If you saw the image on a database, like Artstor:
Last Name, First name. Title of art work. 1980. Museum, City. Name of database. Web. 20 Aug. 2015.
You have a few options to search or browse Artstor.
1. Once you're in Artstor, look at the left side of the screen, you'll find a search box. You could search for a topic, artist, country, type of art work, or even themes.
2. Just below the search box, there is a browse section.
The results page will be small thumbnail images of the artworks
To enlarge the image, just double click on the thumbnail.
Once you have the image up to find out more information about the artwork, click on the i towards the bottom of the image pop up screen.