I've never used Fritzing, but presumably when the image is generated it is stored on their server, so you could just copy the location of the image and use that.
Its a stand alone app there chief
As far as images on the forum I can see it from both sides
As a user it is annoying to find a image host, or like me host it on your own site... course what happens if I for scrapybook decide to hose my server, information here is lost
As an administrator of a fairly active website (that we allow images on) they can start to suck down space, course this can be handled, like on our server we have scripts that compress and resizing images to our liking, but even then at a scale as large as this place gets it sill might not be in the best interest
I would like to see it, its not really that hard to run a image tru gd before stashing it away, and we are not a photography forum so poor image quality would be ok, and much preferred to missing images when I finally snap and go on my killing spree, thus not making the server rent