You don't need a custom iso image, standard Debian Jessie image supports installation over serial port too.
But, you have to give additional boot parameters for kernel which will be used to boot into installation system.
When you see the first prompt, press H to switch help menu. This is required to give special boot parameters.
You will see following prompt:
Press F2 through F10 for details, or ENTER to boot:
Now, give following parameters and press ENTER
install vga=off console=ttyS0,115200n8
If you have multiple serial ports you may have to change
ttyS0 to related serial port device name or baudrate parameters but most of the case, it will be
115200n8 as here.