As per the title, I hope to change the background on MRIcron from black to white. I can't see a setting with which to do this.

(Note that I don't mean the 'background image' of the brain/template, but the black background upon which all other images are overlaid).

I'm using MRIcron to make slices, and have tried exporting images and changing the black background color to white, but this leaves a thick black outline around each image.

Hi!
I was looking for the same. Try MRIcro, the one with the purple brain. You can change the background there.
I hope this helps :-) It worked for me.
Yacila

Yes, The MRICro and MRICrongl can change background, But they have others problem as follow:
1. Theyt can't set three images in a row. 
2.  sets background to white that have some flaw, example the lower right corner is still black in three images model. Can you fix it? Thanks.