What country are you in?
In the U.S., if you told them you have a disability and they refused to make reasonable accomodations for you, they are in violation of the law.
The whole situation sounds fishy. You were working for them on a trial basis, created logos that they are still using, and then they decided not to use you when it was time to decide whether to make things permanent.
Am I understanding you correctly -- were you not being paid? If so, it sounds like they were just using you. I've seen other companies do this: they hire someone on a trial basis for no or very little pay and then when the trial period is up say it just wasn't working out. They have no intention of hiring anyone on permanently -- they just want the cheap labor.
Did you sign anything that says that they have the rights to anything you created while in their employ? If not, you may be able to sue them for a share of the profits from the logos you created or perhaps stop them from using the logos at all.
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Autism Speaks does not speak for me. I am appalled to discover that Alex Plank has allied himself with an organization that is dedicated to eliminating autistic people. I no longer wish to have anything to do with Wrong Planet. Delete this account.