Imported png image transparency problem

[Ofnuts]

Quick test: zoom the image in and out. If:

• The individual squares of the checkerboard pattern keep their size: this is Gimp's own background and the image is truly transparent
  
• The size of the individual squares changes with the zoom level: they are part of the image and the image isn't transparent. On site with paying images the sample images do have the checkerboard pattern baked in and aren't really transparent.
  

You can also changes the background pattern appearance in Preferences  > Interface > Display and see if that changes what you see.