Are you trying to downgrade your iPhone?
The required resource cannot be found: This alert message occurs when your device has a newer version of iOS than what is available in iTunes. When troubleshooting a device that presents this alert message, go to Settings > General > About and check the version of iOS on the device. If it is newer than the latest released iOS version, the device may have a prerelease developer version of iOS installed. Installing an older version of iOS over a newer version is not supported.
Error 1015: This error is typically caused by attempts to downgrade the iPhone, iPad, or iPod touch's software. This can occur when you attempt to restore using an older .ipsw file. Downgrading to a previous version is not supported. To resolve this issue, attempt to restore with the latest iPhone, iPad, or iPod touch software available from Apple. This error can also occur when an unauthorized modification of the iOS has occurred and you are now trying to restore to an authorized, default state.
If not, try to move or delete the IPSW file and let iTunes download it again.
Rename, move, or delete the iOS software file (.ipsw)
iTunes uses "ipsw" files to restore your device. If those files are unusable, then try deleting them, renaming them, or moving the file to a different directory. This will cause iTunes to download a new copy of the IPSW. When a restore issue is specific to a user, it is likely due to an unusable .ipsw file. If removing the .ipsw file does not resolve a user-specific restore issue, then the issue is caused by other user-specific security software settings or iTunes preferences. Creating a new user will also cause new .ipsw restore files to be downloaded in the new user. You can find the ".ipsw" files in these locations:
Mac OS X
iPhone ~/Library/iTunes/iPhone Software Updates iPad ~/Library/iTunes/iPad Software Updates iPod touch ~/Library/iTunes/iPod Software Updates Note: The tilde "~" represents your Home directory.
iPhone C:\Documents and Settings\[username]\Application Data\Apple Computer\iTunes\iPhone Software Updates iPad C:\Documents and Settings\[username]\Application Data\Apple Computer\iTunes\iPad Software Updates iPod touch C:\Documents and Settings\[username]\Application Data\Apple Computer\iTunes\iPod Software Updates Note: To quickly access the Application Data folder, click Start, and then choose Run. Type %appdata% and click OK.
Windows 7 and Vista
iPhone C:\Users\[username]\AppData\Roaming\Apple Computer\iTunes\iPhone Software Updates iPad C:\Users\[username]\AppData\Roaming\Apple Computer\iTunes\iPad Software Updates iPod touch C:\Users\[username]\AppData\Roaming\Apple Computer\iTunes\iPod Software Updates Note: To quickly access the AppData folder, click Start, and then in the search bar type %appdata% and press the Return key.