I’ve used it and at the time it would download the IPSW for you. I’m not sure if there was an option to use your own IPSWs. Also, it is important to note that t

I’ve used it and at the time it would download the IPSW for you. I’m not sure if there was an option to use your own IPSWs. Also, it is important to note that they would charge you for different features so being able to use your own IPSWs could circumvent this pricing model. Of course their boot loader could enforce Apple signing policy but then it would only be able to run Apple code.