Okay, I found the root of the problem - it garbled my sprite when I chose to accept the programs suggestion to repoint the image
Basically, the message was "XX bytes (usually 210+ in hex) is too big. Repoint image to offset $HEB0B20?"
What I'm wondering is firstly how big a sprite can be in bytesbefore this message shows up, and secondly how I can draw a sprite with smaller dimensions and with a simpler colour scheme than the original and manage to trigger this message?
It's really getting on my nerves, because while I can work around it with small pokemon, once I get into evolutions there's no way in hell i'll be able to draw the sprite big enough to make it look anything other than uninspiring, for fear of killing my ROM.
PS: M.L, i agree.