I'm a noob at concurrent programming, but from what I can tell, the only concurrent mutation is when setting the pixels, in image.go:

    94		p.Pix[i+0] = c1.R
    95		p.Pix[i+1] = c1.G
    96		p.Pix[i+2] = c1.B
    97		p.Pix[i+3] = c1.A

IIRC, copying a single byte is safe, so what could happen is that you could end up with weird pixels, if two nodes were setting the same pixel at the same time.