Linking to other sites does not affect transfer.
This brings up another point ... if you have the images stored in another location (say a site that hosts images free), you can sometimes place that image on your page without having to have the image actually on your site. This is called "hot linking" and some of the image sites will not allow it (because it affects their transfer rates).
You also asked about subdomains ... they point back to a
folder within the original site which can make the domain name much shorter and cleaner (for example, bob.fornal.org/Genealogy instead of
www.fornal.org/Bob/Genealogy/). It also provides the capacity to have multiple users/accounts on one hosted site:
1. My site: bob.fornal.org
2. My wifes: jen.fornal.org
3. My brother: tim.fornal.org
4. My sister-in-law: kristin.fornal.org
These subdomains also have the added benefit of looking much more professional. My sister-in-law is intending to use her space as a virtual resume and can post the following on her correspondence:
Resume: kristin.fornal.org
E-Mail:
kristin@fornal.org
... which looks better than ...
Resume:
www.fornal.org/Kristin/Resume.html
E-Mail:
kristin@fornal.org
There are other uses, it comes down to your creativity ... another that I use is configuring: images.fornal.org as a subdomain which allows me to place all my site images in one location and use a MUCH shorter path.