You need the image to be online somewhere. If you see an image on a website, right click it then click "Copy image address". Or if you want to post your own picture you can upload it to http://imgur.com (really simple and quick image hosting) and copy the image link from there.
Then on a question/answer, click the image button (6th from left), put the URL in the box and click OK. It will put a code like this:
![golbat] <-- where the image goes
...the rest of your post...
: http://img.pokemondb.net/sprites/red-blue/normal/golbat.png <-- the URL
And that gives something like this:
Remember, don't go putting tons of images in every post. It's just there to aid in questions and answers.