There is 1 more cause of this error after upgrading Prestashop. It is: did not generate images. Here is what you need to do:
1. Log into your PrestaShop admin panel and open Preferences -> Images
2. Scroll down untill you see Regenerate thumbnails button. Thumbnails regeneration is something you need to do every time you upload images and change image sizes:
3. There you can select a type of images that should be regenerated – product images, categories images, etc.:
4. Click Regenerate thumbnails button and all the thumbnails will be re-built for you.