Dpi/ppi is meaningless on the web, the only thing that matters is pixel dimensions. And your HiDpi images will need to be four times the size of regular images, that is, double pixel dimensions. You cannot do this without scripting, and the best tool around is probably Retina Images.
You will have to declare (regular image) dimensions for your images in the html, and have two versions of the image in your web folder. One regular image, let's say 500 x 400 (image.jpg), and one image at 1000 x 800 (firstname.lastname@example.org). Retina Images will then check the pixel density of the monitor, and serve the appropriate image. This way you dont' have to serve large images to monitors that don't need them. You can see it in use on my website, http://perberntsen.com
And InDesign is not the right tool for the job, you will need to use Dreamweaver.
InDesign measures a pixel as 1/72", so if you make a new document with the intent as web and with page dimensions of 720 x 720 pixels, the page's print dimensions in inches will be 10 x 10" (change your rulers to inches and you will see that). So if you place an image that is 720x720 pixels at 72 pixels per inch it will fit to the page dimensions. If you place a 720x720 pixel image that is 144ppi, its output dimension will be 5"x5" and will take up half the page.
If you export the InDesign page as a JPEG at 72ppi it will open in Photoshop as a 720x720 pixel image, or if you export as PDF and open the pdf as 72ppi it will also be 720x720 pixels.