To cite an image of an artwork in a database like Wikimedia, treat the database as the container of the work, just as you would another online container, such as a museum website. In most cases, you can create a works-cited-list entry for the artwork itself, beginning with the name of the artist, or, for an anonymous work, with the title or a description.
For example, if you accessed Pieter Bruegel the Elder’s Children’s Games through Wikimedia, you might cite the work as follows:
Bruegel, Pieter, the Elder. Children’s Games. 1560. Wikimedia, commons.wikimedia.org/wiki/File:Pieter_Bruegel_the_Elder_-_Children%E2%80%99s_Games_-_Google_Art_Project.jpg.
Multiple images of this painting exist on Wikimedia; providing a URL for the commons.wikimedia.org page instead of the upload.wikimedia.org page—especially if your project is in a digital format with hyperlinks—will allow readers to access more information about the file you used.
If you reproduce an image that is not in the public domain, be sure to give credit to your source and provide a note on permissions. See this post for more on giving credit for images.