As this StackOverflow answer states, you really shouldn't atempt to parse [X]HTML with regex. findstr has very limited regex support in any case.
Use a proper HTML scraper/parser like Xidel instead. A command like the following will do what you're looking for:
xidel <URL or HTML file name> -q -e "//a/extract(@href/resolve-uri(.), 'https:\/\/s-media-cache-ak0\.pinimg\.com\/originals\/.*?\.jpg')[. != '']"