The short answer is:
Unless you are really lucky, your request is unfeasible.
The long answer is:
If you goal is to get familiar with protocols and data flow, I'd suggest you start studying the CCSDS standards, as they're the usual starting point for telemetry and telecommand formatting. Then, I'd suggest you find some software that will help you get acquainted with simulating the dynamics of orbit passes, access time and so on. STK has a free version you could check out.
This would give you the basis on what you should know before talking to someone who actually manages a cubesat.
Now, here is the tricky/costly part.
If you own an amateur radio station (which is not free), you can probably receive telemetry from many cubesats, which send some un-encrypted data to allow their orbit to be tracked by many sources. I know at least NanoSatC-Br1 allowed this. As this page quotes: "the project is very much dependent on information provided by the amateur radio community."
If however, you want to send commands to a spacecraft for free, you are in bad luck. This is like asking to use some laboratory in a university to which you don't belong (or maybe you do, depends on you).
Note that not so many cubesats can/could actually relay images to ground. Asking for images from a commercial cubesat (such as PlanetLab's) should be easy, but not free, and you'd hardly have direct control over it. Asking for an image from a scientific/research cubesat could be free, but difficult. They'd also be wary of you sending commands to their satellite.
Also, if you are not willing to pay for it, I'd guess you are also not willing to offer any collateral. What if your commands actually cause the cubesat to become un-operational? Hardly any researcher would disclose such concern, but launching a cubesat is hard work from a funding point of view, and nobody wants to throw it out the window.
Not sure what your intention is, but your best bet is likely to join or cooperate with some university that is currently developing a cubesat.