I’m developing a custom WordPress theme. When I go to select themes in the Appearance menu, my theme does not have a preview image. Is there something I need to do to get a preview image to show up? I can’t find any resources on the web that talk about this. Maybe I’m just searching the wrong thing though…?
There is no automatic preview. You need to create the screenshot yourself and place it in the theme/child theme folder named screenshot.png
The recommended image size (currently) is 1200px wide by 900px tall, however the screenshot will only be shown as 387×290. It is over-sized to allow for high-resolution viewing on HiDPI displays. Note that as of version 4.7.4, the bottom ~18% of this image will be cutoff so keep logos or other graphics centered.
For the latest info see:
http://codex.wordpress.org/Theme_Development#Screenshot