Our app will not support this type of formula field merging multiple images. Our app supports image merge; however, the image field should contain a single URL to the image. With some additional development, you could create a workaround process to support your use case. For example, create a new object called ‘Image’ or something similar. This would be child object of your current object in use for document action. The new Image object would contain the URLs to the images stored in your formula field. With this, you could do some kind of synchronization between the formula field and this child object to get all URLs. It would be easy to merge images, since every image would have a separate URL.