Select identical field from row above current

So basically I have an image URL displayed in an iframe.

The image URL is queried from an SQLite database using CGI (Python), and imagine it looks like this:

ImageID:

image1.png
image2.png
image3.png

If image2.png is currently loaded inside the iframe, how would I create a button that always links to the next image in the database (image3.png)?

So if the current loaded image was 'image1.png', the 'next' button would display 'image2.png' inside the iframe. However if the current image loaded was 'image2.png', then the 'next' button would display 'image3.png'