Nero, Alcholol 120% and some other softwares produce iso files, they just use different extensions, like .img, .nrg or .mdf. Despite the different extension, they all are ISOs, so DVDFab will read and process them. Just the stupid file extension filter of the "Open Image..." function of DVDFab doesn't include those other extensions. So one needs to rename the extension of the images to iso before DVDFab shows them in the "Open Image..." dialog.
That's very uncomfortable and could be fixed by the developer in about 30 seconds. I just hope he reads this forum!