I've only had to sign one, and it was for a photo contest that I won $1000 with a photo of my cat and I believe it limited their use to that photo (which I was fine giving up, since it was a "pose your pet with our product" thing). As far as I know, they used it to promote the contest while it was still running (I was listed as a recent winner with her photo adjacent) and they posted a "winner's gallery" on their facebook page once the contest ended.
I'm still waiting for the day I randomly pop into the local store in the chain that cosponsored and see it on a banner declaring me a local winner or something crazy like that, but I'm not counting on that to happen unless they run the same sweep next year and use this year's winners to promote it next time around.
I'd probably be a little more hesitant if it were my likeness, but my level of hesitation would vary depending on the prize being given. I did give permission to use my likeness for a recent $100 gift card win, but it wasn't as formal as a publicity release, it was handled in email with the sponsor and I chose the photo he posted on the contest page on his site.