I don't claim to know what I'm doing, so please take the following with a grain of salt. I've been in the same boat fighting with this issue the past couple of days. I created this account just to respond to you (how special you must be) and maybe help my fellow GS3 brothers and sisters. This is a temp account so I may not respond if you have any questions.
I have a Verizon Samsung Galaxy S3 v4.1.1 ROOTED with the stock ROM. I renamed my .iso and it still came up but at least there was no installer, just like a blank cd rom. Still waited for a few seconds before it switched over to MTP.
What seemed to work for me is the following:
1. Type the following but don't press send *#22745927. This should let you enable the special hidden dialer menu settings. Enable settings
Note: All over the net I read were people said to just type a number sequence and it would let you access special menus, yada yada, but it never did anything for me. I just found the above code about 10 minutes ago. Afterwards those codes worked
2. Type *#7284#, select the Qualcomm USB Settings and choose MTP+ADB. I originally chose just the MTP but it did nothing for me, YMMV.
The only thing I can't figure out is how to change it back. I don't care to put it back to "normal" but it would help me understand what exactly that I changed made it work. Been messing around all evening trying this and that so there might be more to it. I hope this helps, maybe at least get you going in the right direction.
Mike