@kainu7 - I had the same problem learning to post photos here not that long ago. Here's what I learned:
If you go to Flickr and post photos there, you can then, under each photo, click the down-arrow where it says "Share" above the photo, then choose the "Grab the HTML/BBCode" subsection. At the bottom, select BBCode and the picture size you want (I choose Medium 640 but pick whatever size you want). Then copy and paste the full code snippet from the box as it appears on the screen from Flickr directly into the message text you're typing. Your picture will then appear, once you post it, as part of your message instead of as a link, which as a number of people here mentioned is considered insecure and a possible avenue for malicious individuals to try to induce viruses to others' machines.
Hope you find this info helpful!