The app requires Javascript to drag-n-drop and to "Select" your photo? If neither is working, than Javascript is not enabled or being blocked in your browser?
If the app gets stuck on "Preview" message graphic, just note that it can depend on your net connection upload speed. The slower your connection, the longer it will stay on Preview until finished.
The other issue is that IMGUR servers could be unusually busy or doing maintenance, which will either slow uploads or even cause it to get stuck. In which case, close your browser and restart the app. You may want to wait a few minutes for any issues with IMGUR to clear.
Drag-n-drop has only been supported in IE since v10. It apparently doesn't even work on IMGUR's website if using IE. Firefox is the most compatible browser with least issues in this regard.