Our app will not support this type of formula field merging multiple images. Our app supports merge of images however, image field should contain single URL to the image.
With some additional development, you could create a workaround process to support your use case. One example could be creating new object called ‘Image’ or something similar. This would be child object of your current object in use for document action. Then this new Image object would contain URL to the images stored in your formula field. Then you could do some kind of synchronization between formula field and this child object to get all URLs. And then it would be easy to merge images as for every image we would have separate URL. For more assistance and to talk through your use case, we offer premium app support.
You can review this FAQ for how to create a formula field.