Windows 8 by default opens any image in Photo App as it comes as the default program. If you are used to editing photos using Windows Paint and want to make that as the default program for editing images then you can follow the below mentioned steps.
Step 1: Open the folder that contains images files which needs to be edited using the Paint.
Step 2: Right click on the image, navigate to Open With option and select Choose Default Program from the sub menu
Step 3: Select Paint for How do you want to open this file.
By this way you can make Paint as the default program for editing image files.