[Bulk Post Creator] Featured images not showing

Hi, I am using the Bulk Ai Post Creator with royalty free images as featured images. The posts that are being created have a featured image assigned to them, but that image cannot be seen. It is as if the image was not properly uploaded and if I go to the image URL there is nothing. In the media library I will see the featured images just as grey squares, like the screenshot I am attaching. My server's WP Memory Limit is 256MB, PHP Time Limit 900 and PHP Max Input Vars is 2000. What is happening and how can I fix it? Thank you