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…?
R/. There is no automatic preview. You need to create the screenshot yourself and place it in the theme/child theme folder named screenshot.png http://codex.wordpress.org/Theme_Development#Screenshot
For wordpress.org themes, the screenshot should be reasonable facsimile of the theme, and should be 880px wide and 660px in height (any 4:3 height:width) screenshot will be fine. To add a screenshot, create screenshot.png file and add it to the root director of the theme. e.g wp-content/themes/theme-name/. You can use screengrab, awesome screenshot or any other firefox addon or chrome extension to take screenshot and set height and width in photoshop. You can also use built-in feature of firefox for this reason.
Click on alt button on your keyboard and you will see a menu at the top of the browser, Click on tools > web developer > responsive design view, now set custom height width and take screenshot.