Related: http://octobercms.com/docs/themes/development#preview-image
The theme preview MUST be placed into assets/images/theme-preview.png. This is a very annoying limitation.
In our case, we're using an image minifier (with Gulp) which minifies everything from assets/src/images to assets/dist/images. It would be silly to create a new directory, just for the theme preview image.
Fixed by #2426
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Fixed by #2426