If you were powered all the time, the simplest would be to run a browser that uses refresh:
Code:
<meta http-equiv="refresh" content="1"/>
It could use a local page with a remote image.
You could have it hit an HTTP server you set up that would change its content every hit.
You could have it hit your Google Drive or whatever and just push the photos yourself.
A simple app could do the same thing, be more robust and lightweight than the browser.
Once you get into sleep things become more complicated.