Paint 3D was designed with 3D dioramas in mind, but opening an image with the Edit with Paint 3D shortcut will bypass the 3D setup and take you into the 2D editing tool. For the purposes of editing a photo, you’ll want to ignore the majority of the Paint 3D UI, anyway. You’ll need to left-click once to preview the photo, then right-click it, drop down to the Edit and Create sub-menu, and on to Edit with Paint 3D. In Photos, this option isn’t immediately available. With the latter, simply right-click the file and scroll down to Edit with Paint 3D. The easiest ways to jump directly into editing your photos are either to open the Photos app with your stored photos or open the folder on your PC where you archive your photos.