How do I get the real file path of an image?
The second one is the technically correct path. The first one, /phone/..., is just the Gallery app simplifying the display and telling you that the folder is in phone storage (instead of external storage).

When connected to the PC using MTP, the root folder is /storage/emulated/0/ (you only see the contents under it), so you should be looking for the part of the path after it, which is Pictures/Messages/imagename.jpg.