what r u talking about? there r some custom firmwares avaliable on the webs and iTunes accepts them without any problems....
i didnt manage yet to update iOS to a more recent version,the answers given here do not work..
Then by all means, download them and utilize them. Custom IPSWs in the true definition are unique to device, so even if you throw them on the web and download them, you can't use them. DFU IPSWs do not count as custom IPSWs, they're just re-packaged IPSWs. Sn0wBreeze stitched pre-modified IPSWs do not count as custom IPSWs either. Baseband preservation-only IPSWs that you find on the internet are made at the time when that iOS was the latest iOS. You cannot re-package an already re-packaged IPSW, so if you wanted to preserve your baseband while upgrading to an iOS that's not the latest iOS supported by Apple/iTunes, the ones you download from other sources will not work since they have no blobs/APTickets.