Forgot, I am using CS6 InDesign
(may I ask here - The publisher has told me that I have to also convert photos to CMYK, can I do them at the end all together (if so how?) or do I have to go back and do each pic individually?) thanks
Open up the Find Font dialog (Type > Find Font), does it show the font helvetica there.
It could be that that font is embedded in the images that you have in the document. (or was used in them but not embedded hence sort of missing)
It happens only on Photo pages, not on other pages? ID uses Helevetica for the automatic page information marks if you specify them in the export options, and if you have glyphs in the filename that can't be represented that might be a source of the problem, but I would expect this on all pages.
You can convert all othe images to CMYK during export. The quality of the conversion will be the same as if you open the image in Photoshop, choose Image > Color Mode > CMYK and save. If you want to do any color tweaking, or any sharpening or other adjustment you need to do them one at a time.