I followed the beginner steps for creating a child theme. The child theme was created and it appears as an option on my Themes page. But unlike the other Themes on the page (including its parent), there is no preview. The other themes (“Twenty Twelve,” “Twenty Thirteen,” and “Twenty Fourteen”) are all listed with an image preview. My child theme (“Twenty Fourteen Child”) does not have any image. The child theme can be applied without issue.
This issue arises when I create child themes from other themes as well. Any ideas why this is happening and if I can resolve?
Below are the methods you can try. The first solution is probably the best. Try others if the first one doesn’t work. Senior developers aren’t just copying/pasting – they read the methods carefully & apply them wisely to each case.
Unfortunately this isn’t covered in that codex page. These previews of the themes aren’t auto generated. You will need to take a screenshot of your theme, and upload that screenshot to your root folder as ‘screenshot.png’. This screenshot will be automatically used then as a theme preview pic for your theme.
I quote from the Theme Development page in the codex about screenshots for theme preview pic
Create a screenshot for your theme. The screenshot should be named
screenshot.png, and should be placed in the top level directory. The
screenshot should accurately show the theme design and saved in PNG
format. The recommended image size is 880×660. The screenshot will
only be shown as 387×290, but the over double-sized image allows for
high-resolution viewing on HiDPI displays.