Release images are static and aren't re-released unless there's a serious bug on the installer. This is reflected in the dates of the images, as seen on the Releases site. There will probably not be a fresher ISO for 17.04.
LTS versions usually get point releases (eg 16.04.1, 16.04.2 ..). Sometimes these will just be updated versions of their predecessors but they often also include newer kernels. There's no hard rule on this so check release notes.
Versions in development will have "nightly" or "daily" images available, although these are really not suitable for day-to-day use. Don't assume that everything is latest and most secure, that's often not the case.
The graphical installers usually have a "download updates" option which downloads newer versions for installation. You're still booting into an older version to do the install though.
And 16.10 isn't a great candidate for staying on something until you find another Ubuntu release because it leaves support this July. I'd suggest the 16.04 base release (not the 16.04.2 point release which probably has an updated kernel in it). Doesn't fix your ISO security paranoia that might be driving this question, but if you can update from installer in a non-hostile network, I don't think there's a problem.