IANAL but I am a stock footage contributor and make a nice bit on the side each month from it.
Image releases are fairly common and are used to release people and locations for use such as stock photography, but also for film and TV etc. Basically you need the person's written permission to use their image for your own production or financial gain (depends how the contract is set out).
Often you will also need a Property Release for somewhere you can recognise and is the main subject of the image. For example, if you took a skyline shot of London with the Houses of Parliament in view, you'd not need to release this as it forms part of the view of London. However, if it's a shot of just the Houses of Parliament, slap bang in the middle of your photo and not much else, you're going to need someone with some authority to sign off your photo for resale.
These days you seem to need a release for a lot more, such as animals. Especially horses and dogs. Basically you could be taking a photo of a champion horse and making money from industry stock photo buyers who know the horse, but the stock agency doesn't. So they will usually ask for a Pet Release...
Which leads me on to cakes. The best advice I can offer with this is to get a property release from the bakery and be straight up with them that you want to use their cakes for stock photography. Having a release is far better than not and you may find that a stock agency adds a note to your account saying that the release isn't necessary for future uploads.
As a photographer and filmmaker I don't advocate doing work for free, but when it comes to stock it's a bit different in my eyes. I've offered someone a free photo of their house for letting me use their garden for filming stock footage. It's not much time out for me, but is a means of payment for something I could earn money from if my stock footage idea for that shoot works out. So I would offer the bakery some imagery for their instagram or social platforms and not use those images on the stock sites. You're sacrificing a few photos out of hundreds to keep your bakery happy and you in return get as many photo opportunities as you like.
You can get a footage release from here https://submit.shutterstock.com/legal
On a side note, welcome to the stock photo/footage industry. You're off to a great start if you've already got a niche. Contributors with niches usually do a lot better. However it's worth noting that you won't become rich overnight and you'll need to work hard on uploading a lot of photos before you'll start seeing regular sales.
Good luck!