In 500px, on the right hand side of the screen after you've clicked to view one of your own images, there is an "Embed" button. Unfortunately, it's HTML and they don't have it for BBcode, so copying and pasting that link won't work here. The easiest thing to do would be to copy the address to the .jpg out of the HTML link they give you, then put [IMG] tags around that here (or hit the "Insert Image" button and paste the link in there).
For example, copy the bolded address from your links:
<a href="http://500px.com/photo/5219736"><img src="
http://pcdn.500px.net …514154636536cc93e8e/4.jpg
" alt="Sunset Cruise by Shaman Tanthew (nathmath) on 500px.com" border="0" style="margin: 0 0 5px 0;"></a><br/><font style="font-size: 120%;"><a href="http://500px.com/photo/5219736">Sunset Cruise</a> by <a href="http://500px.com/nathmath">Shaman Tanthew</a></font>
Someone else probably has a better way.