Just some thoughts..
1. Have you put the picture into the correct images section and not as an object?
2. Are you using the Display Image (on screen)?
3. Try using the X/Y zoom functions for the picture to make it larger or smaller. I cant remember if there's any other function for resizing pictures for display.
4. You can always try re-uploading the picture into you project for the exact dimensions of the game window size, edited in another program to exact dimensions.

I didn't find this zoom option, but it's ok, the image was 4k and it simply didn't fit in a place that wasn't even very large and I didn't want to resize it and run the risk that if the game was run on a smaller screen it would cut out, what I did was load the image as a sprite then use the editor to place the image in the size of the screen so regardless of the resolution chosen by whoever plays the image remains appearing entirely