I think it depends on the design of the image! If the image is created with text in mind, then you could leave enough room around the main subjects to add the text in without it getting in the way of the action. In that case, text inside of the image is great!

Otherwise, it may be best to keep text separated in order to fully appreciate the entirety of the image.

So in conclusion, either or! Depends on how the image is spaced out!

Thanks Kaden! I appreciate your feedback. Until now I have designed my images with details which would make text bubbles a bit messy. So I guess I have to change the overall design to make it more suitable for comic dialogues.