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?

I heard that dynamic URLs are harder to crawl - but not impossible.

Here is a good article to read: Dynamic page challenges

According to Google, crawling and ranking your dynamic URL that is made to appear like a static URL may be quite difficult. A dynamic URL normally has a question mark and an equal’s symbol. Here are some URL optimization tips:

-Don’t make the URL structure too complicated or difficult to manage.
-If the URL has parameters that are not related to the content, get rid of it.
-Try as much as possible to keep the length of the URL short.
-See that the keywords are placed at the beginning of the URL.