mhhh, I'm not sure, however, I don't think there's such a function in SFML (it's just not what SFML is designed for in the first place
)
But I think that scaling the Image (respectively the Sprite) in SFML will give you the same result as scaling it with any other program...what counts, is the size of the original image, e.g. if you want to resize from 10*10 pixels to 800*600 pixels, the result *will* indeed be rather ugly
edit: trying what I just wrote, I must say that it in fact does, what I predicted...the quality of my scaled images is just as if I had scaled them with paint or whatever...
However, I came across another problem: with my "workaround", you can only scale images up to you actual screen-resolution, because for any reasons, RenderWindows won't display if they oversize your screen...