Anyone in Los Angeles county that would know about parking in alleys?

I was parked in an alley in the city of Compton because it was street cleaning day so I couldn't park on the street, so I got a ticket for parking in the alley but there was no signs anywhere saying that there was no parking permitted in the alley, I contested the ticket and send proof (pictures) that there were no signs n still lost the contest. I had to send full payment of the ticket n they were going to conduct a further investigation and if they were at fault they were going to reimburse me for the full amount. So after the investigation according to them I was at fault, so my question is how was I supposed to know that when there are no "no parking signs" in the alley or outside of the alley, can they do that even when I have pictures showing no signs?

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