No credits are spent for sending out emails or using the Drip Campaigns in any way EXCEPT when using the personalization option with the Image Editor.
Therefore, for every opened email that contains your personalized image, 1 Credit will be deducted. That means, if you've sent your email to 50 people, but 30 of them opened it, 30 credits will be deducted.
Note: If your credits are spent in the meantime, the next customer that will open your email ❗WON'T be able to see the personalized image❗
No credits are needed for using the Email Warmer!
Check related articles for more info on other limitations within the Drip Campaigns!