Elements 15: When I open a pic in the edit mode, the pic opens in a small box in upper left hand corner

What OS/version are you running on? Can you post a screen shot of your entire screen to see what you're describing?

There is a work round to this as I found out. I was having the same problem. It only happened after the last Windows 10 Update. If you close Elements and then hover over the elements shortcut then right click and choose run as administrator. A box will pop up saying do you want this app to make changes to your device. Click yes and elements will open and run normally.

See:
https://feedback.photoshop.com/photoshop_family/topics/floating-all-windows-elements-15
This known bug has been solved in PSE2019.
However, I don't think the fix has solved the core of the problem when you have allowed 'floating windows' in your preferences.
Now, when you open your files in floating windows, you can see that they are opened in the top left small windows and finally the editor triggers automatically the 'float all' command, which opens the real windows.

The common workaround was to issue that 'float al' command from the layout icon on the bottom bar.