After considerable research, I am none the wiser. It looks like in theory a subdomain site should work with either sub.domain.com or domain.com/sub though such appears not to be the case in reality. I can only get the above site to work with the domain.com/sub format. I have another concrete5 site, also ver. 220.127.116.11, in which some of the images and layout work correctly when accessed by http://myecwidstore.tanneco.com/index.php/services... whereas other images and layout only work correctly when accessed byhttp://tanneco.com/myecwidstore/index.php/services... . With the sub.domain.com version, the Title Logo is missing but the Client pictures at the bottom of the page are present. On the domain.com/sub version the Title Logo is present but the Client pictures at the bottom of the page are missing.
Any assistance would be greatly appreciated as I would certainly prefer to stop doing whatever I am doing wrong as opposed to reloading images and such every few weeks. Incidentally I placed a redirect in the htaccess file for the picsnmore site so that anyone using the failed sub.domain.com link would be redirected to the working domain.com./sub version.
Thanks again for your support.