Make BootCamp work without external drive
Updated Answer

The 2012 and newer Macs are installing Windows 10 to boot using EFI/GPT method. The older Macs generally only have firmware that allow Windows to boot using the BIOS/MBR method. If your MBP requires the use of the this legacy method, then even if you can trick the Boot Camp Assistant into an iso only install, your computer may not boot Windows properly. Furthermore, the Boot Camp Support Software will not be written to handle the new EFI/GPT type of booting.

BTW, install Windows 10 using VirtualBox is not a trick. The process is exactly what the Boot Camp Assistant does for the newer Macs, with some additional steps. The newer Macs use the EFI/GPT boot method and therefore do not need to change the MBR in order to boot Windows 10. For the older Macs, using the BIOS/MBR method, this is a requirement. The purpose of VirtualBox is to copy the installation files from the iso file to the partition from which the Window installer will boot from. This is exactly what the Boot Camp Assistant does for the newer Macs. Once the files are copied to this partition, you can boot the Window installer just as one did using a DVD or USB flash drive.  Once you boot the Windows installer, VirtualBox, as well as, the Boot Camp Assistant no long play any role in installing Windows.

Original Answer

Generally installing directly from the iso using the Boot Camp Assistant is reserve for the newer Macs (2012 and newer). Also believe you need Windows 10, but Windows 8/8.1 might work. 

In any case, your MBP is (13-inch, Early 2011). The accepted method for installing Windows is to use your optical SuperDrive. If your SuperDrive does not work, you can still install Windows, but you really have to want to. The procedure has many steps.

You did not specify which version of Windows you want to install. I believe you can install 7, 8 and even 10 if you wish. Without an optical drive, you will need to install a free copy of VirtualBox. If you wish to proceed, update your question to include which version of OS X you are using, what version of Windows (7-8-8.1-10, home or pro, 32 bit or 64 bit) you want to install and if you intent to use the free Microsoft upgrade to Windows 10.

Also, did you replace your SuperDrive with another internal SSD or HDD? If so, do you intend to install Windows on such a drive?