To make Photoshop your default image editor:
Windows: Open any folder, right-click on any image, and select Choose default program… Then click on the Photoshop icon, and click the OK button. You have to do this for every single file type (i.e. .jpg .gif .png) that you want to open by default in Photoshop.
Mac: Click on any image and press Command I to bring up the Info dialog box, and in the Open With drop-down choose Photoshop, and click on the Change All button. You have to do this for every single file type (i.e. .jpg .gif .png).
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