On the Spectrum side, you can use any of the graphics programs available on WoS. The following link points to all the art software found there - I recommend using something like Art Studio. Spectrum Graphics Software

If you want to edit SCRs on the Amiga, my favourite way to do this is to obtain ZX Datatype from Aminet, and load SCRs into Personal Paint (or other art software) for editing. You can then save them out, and reconvert them using ZXDT palette matching and no dithering.

Alternatively, the Spectrum Graphics Editor is a nice way to edit images. The Amiga port is available from here.