I did a little researching and it seems that the idea of image rights as we know it in the UK simply doesn't exist in that form anywhere else - it's a unique deal which gives the clubs far more control than they would have anywhere else. Still, I do know that MLS is very, very precocious regarding its control of intellectual property, and I also found a story from a few months back where MLS signed a deal giving a specified company the sole rights to produce certain items of merchandise with player images on them, so clearly there will be some image rights agreement.
What it seems like to me, and this is slightly conjectural, is that the league holds all image rights, but doesn't use those rights as widely as clubs do in England. I don't think players get any sort of financial remuneration for their image rights - it's simply taken as a given that MLS can control that stuff and that as a contracted MLS player, you agree to let them handle the images as they see fit. I suspect that MLS has more control over which player can be used in TV/magazine/billboard/etc advertising for what product, too, but again that's speculation.
It's a bit of a half-answer, but I hope that gives an idea.