There is no problem in loading a 7.0.x image coming from 8.2.1.
You will at least have to configure an interface, and use a tftp server to get the image from.
You can do "copy tftp flash:" and follow the prompt.
Once you have the image in flash, you configure the boot command like "boot system flash:imagename_file_name", and save the configuration by doing "write memory"
Regards,