If you have photoshop you can expand the canvas to incorporate enough white to fill in the grey bars. If you don't use Photoshop that may make no sense. If you have photoshop and it still doesn't make sense. Try this:
After cropping, go to Image / Image size ... and make a note of the width
Go to Image / Canvas size ... and set the width to the image size multiplied by 1.5 (make sure the units are the same - pixels or centimetres). Set the height to the same value as the width and the canvas extension colour to white.
Now you should have an image that will completely fill the square that Blip have allocated to the journal image. I'm sure they'll get round to set the background to white instead of gray.