If I understand correctly that you have one detail per page, which is displayed in the Page Header, and you have nothing in the Details section, you could do the following:
In the Detail section, insert a second Detail second so that you'll have Detail A and Detail B. Leave Detail A blank. Put the Terms & Conditions image into Detail B, go to Format Section for Detail B, and check New Page Before
. The reasons for the two detail sections is sometimes CR doesn't respect New Page Before in a single Detail section.
Now, go to Format Section, and for the Page Header enter the following formula in the Suppress (No Drill-Down) formula editor:
PageNumber mod 2 = 0
You will now have one record printed in the page header, then a page with the watermark, then another page with a record in the page header, then a page with the watermark, etc.
You'll just need to make sure when printing to use full duplexing.