Open Unity and click "Edit > Preferences" and then click on "External Tools". Click on the drop-down list next to "Image application" (labelled "Open by file extension" by default) and click "Browse".
Browse to where you installed Photoshop (it should be in the "Program Files" folder, I installed it onto my D Drive so the path is "D:\Program Files\Adobe Photoshop CC (64 Bit)") and select "Photoshop.exe" and click "Open".
"Adobe Photoshop CC" will now show in the "Image application" drop-down.
Close the Preferences window and now all image files will open in Photoshop.