This is a deficiency in the current INI file design.
The problem is that the INI file is processed on every page of a multi-page form. So, if you make a field required in your INI file, it's required on the submission of every page of the multi-page form.
And that's the problem you're hitting.
The workaround, until we implement a more targeted INI file design, is to use conditions instead of required to target the last page.
Since every page except the last uses "next_form", that's a good field to trigger the condition you want.
This code in the INI file should work for you:
What this says is: "if 'next_form' is set, do nothing, else, imgverify must be set".
conditions1 = ":@ @IF@ next_form @@ imgverify @ You must enter the image characters@"
Let me know how you go with this.