@Wena I tried out your suggestion, the image was somewhat improved but really only marginally.
I think I'm setting the image parameters in GIMP correctly. I thought that if the image was larger than required in Scribus and with a fairly high resolution that should avoid any jagged edges. The png image when viewed in GiMP is fine, similarly for the Image Viewer, for both png and svg. Even with high levels of zoom, both images look good.
From my reading of the Scribus Wiki, png images are OK, preferable in fact. So, perhaps the problem lies with my settings in Scribus.
Thanks for your help