ImgBurn won’t be able to save all discs in ISO format, like when multiple tracks are detected. In this case, you might see a warning that your image will be saved in BIN format instead.

Will ImgBurn create bootable ISO/IMG files from floppies like the OP requested? In any case, I believe either WinImage (nag/no time-bomb) or Ultraiso (time-bomb) can perform the task.

Need installation media to reinstall Windows? Here's how to create a bootable CD, USB, or DVD with an ISO from scratch!