I have a requirement whereby all imagery is 300x300dpi tiff images. I am creating a html page, which I want to use 72dpi images from the original uploaded high-res image. I do not believe tiff is compatible with all browsers? Is it possible to reference the low-res image if I create one from the high-res, and use the high-res when I do a pdf?

Yes, many image conversion tools can create low-resolution images in a different format, eg. convert high-resolution TIFF to low-resolution PNG. This may require modifying your HTML though.