SEO and Dynamic websites - question

I've been reading about SEO for my dynamic website. From wt I learned, dynamic URLs are not Crawler friendly and usually crawlers ignore URLs with queries and ?-& signs.

My site is dynamic and is a small stock images site. When I search for my images I get results for my works posted on Istockphoto.com and Dreamstime.com but not from my website, although I have friendly URLs and submitted xml sitemap to the search engines.

My question is... how comes istockphoto.com and dreamstime.com are being well indexed despite their dynamic URLs, same things with forums.. how comes forum posts rank high and get indexed despite their dynamic URLs...

Anyone can help me understand this?

well..dynamic urls are not seo friendly but they do index these days. SE have handled this problem by upgrading their algorithms for this prob. but that doesn't mean that one shouldn't use seo friendly urls. these are always friendly and highly appreciated by users and SE. Now,your specific question is regarding about 2 sites. These are quite old sites and have so many backlinks. domain's age and backlinks play a vital role in PR.