It's a lovely LO!!

I agree a touch of red would be good - why not mat the heart on a piece of red card and cut it 3 or 4 mm/quarter inch round so the heart is bigger, overlapping the photos a bit more. For my own preference, I'd make it quite a deep blood red, not bright scarlet. I'd also either ink the heart with black or draw a thin black line a mm or two in from the edge of the heart.

TBH I think the paw print might would detract from the real paw prints in the snow......