fix for #792 (crash on attaching image)

createScaledBitmap returns the source image if height and width of the scaled image are identical to source. So it's not a good idea to recycle the source...

I'm not 100% sure if reassigning roughThumbnail is unproblematic. 
Alternative would be to check if scaledThumbnail != roughThumbnail before recycle.

This change fixes #792 for me. Tested even with the original white test image sized 640x539 :-)

(my first try on a pull request - hope everything is correct)