Well, I thought I had got IH3 running fine on a development server but can't say for sure because my mind is a bit elsewhere at the moment.
There is the issue with sercurity on 1.5 but I didn't think that this actually prevented things from working. Rather people were being encouraged to code to meet those guidelines. But this is just what I believe.
However, there are other issues with making a module work with a new version. The code was written to be compatible with a previous codebase. If there are incompatibilities with the new codebase then that will cause functional issues. To be honest I would guess that that is the case here, or there is simply an installation error.
Having said that these links are actioned using a get parameter on the URL. Which is one of the things that is being discouraged in Zen 1.5. So perhaps there is stronger security in place to prevent this happening. You'd have to investigate or ask the developers.
What happens if you add this to the end of the image handler URL
?page=about
so it reads
image_handler.php?page=about
That should emulate the link that is being used and might shed some light on the issue.
I know there are efforts to get IH working. As Diva knows I am not involved in that. Partly, because others are doing it and partly because I think that IH is overly complex for the needs of most stores. So, I am not keen to spend too much time investigating this quirk.