I have an Ilustrator .ia image that I'm using as a backround for an Indesign page, but when I export the file to PDF the background dissapears completely, is there some exports settings I need to change?
Export your pdf to PDF/X-4 standard and view the pdf in Acrobat or Reader. Set your Acrobat or Reader Page Display preference for overprint preview to "Always".
If you are using a spot color in Illustrator, change this to 4C, unless you are actually printing a spot color.