Using Kadence Cloud there is a built in method to generate thumbnails for you using a screen shot generator service. Please note that thumbnails are just the posts featured image so you can create/upload your own thumbnails without needing to generate if you wish.
API Flash offers a free plan with 100 screenshots per month. Since each post only generates a screenshot once and then is saved on your cloud site it’s unlikely you will need more than the free plan.
Step 1. Create an account with API Flash
Step 2. Add a new API Flash Access Key and copy the key.
Step 3. Add your access key into the Kadence Cloud Settings
Step 4. Generate Thumbnails for your cloud library