In order to take an image from a previous step (like your Trigger, for example) and add it as a featured image on your post, you have to use 2 WordPress steps.
First, you use the 'Upload Media' step to add the image to your WordPress website:
Then, on your 'Create Post' step you make use of the image you just uploaded, by selecting "Use a Custom Value" for the 'Featured Media' field and then using the "Attachment ID":
In order to take an image from a previous step (like your Trigger, for example) and add it as a featured image on your post, you have to use 2 WordPress steps.
First, you use the 'Upload Media' step to add the image to your WordPress website:
Then, on your 'Create Post' step you make use of the image you just uploaded, by selecting "Use a Custom Value" for the 'Featured Media' field and then using the "Attachment ID":