yes a new image should stop it booting as it's got a new boot file. if you use ifa you dont really need a null modem cable but comes in handy to spot faults and see the process. just use a cross over cable and all should be sorted mate. you dont need it to boot right up to use ifa
have you tried holding the up arrow and power button on front of box then letting go after you've clicked ok on ifa?, with power going to the box ? or not ? ( thanks this tip may come in handy, what does it do man )
ok so you've set the ip addy on the network connection
removed everything from the back of the dbox apart from the crossover cable.
wait for the program to open another screen.
and then connect the power