This is usually due to one of three things:
-Insecure / mixed SSL content on your site. Install the Really Simple SSL plugin and see if that fixes the issue. Try this first before the other steps. https://wordpress.org/plugins/really-simple-ssl/
– You’re using a plugin that messes with images. Deactivate all plugins except WC Vendors, WC Vendors Pro and WooCommerce and test.
– Your vendors are not being properly logged in so they have the proper permissions. Make sure they login on the woocommerce /my-account/ page… then test.
You can also try resetting the vendor & pending vendor roles: wp admin > woocommerce > system status > tools: Reset WC Vendors Roles