How to edit ISO Images (Including Bootable ISOs)

Solution 1:

ISO Master

I have used ISO Master to add files to ISOs before. It is suitable for what you require because it retains the state of whether or not the ISO is bootable. I have used it in the past to add other content (like music) to a live disk. Note, however, that after making changes to an ISO file, you can only "Save As", i.e. another ISO file will be created, so make sure you have enough disk space for both.

Solution 2:

PowerISO just released a free Linux version of their ISO editing software¹. Like ISO Master, it retains the state of whether or not the ISO is bootable. However, it allows you to save directly to the original file (by deleting it first), so there is no need to have enough disk space for both files. I also find its interface to be easier and more intuitive to use than ISO Master.

¹ Unlike their Windows program, the Linux version free to use, without any size limitations or the like.