Dunno why it's resizing as they're a decent size on Flickr...
I can see the haloing on Flickr. This is happening because Gaussian blurring smudges the pixels together so it's dragging the white pixels from the petals into the background.
What I would try would be to select the flowers themselves, feather your selection (this will feather "outwards from the flowers" and then invert it. Tjat way the selection "fades away" as it gets closer to the flowers. Then try blurring that.
If you do it the other way round (that is, you select the background and feather it) you will feather into the flowers. It's difficult to explain - does that sort of make sense?!
