I have trouble as well but here is how I do it. Someone may have an easier way but I put the image on Photobucket. Then I copy the image code and when I want to add it I paste the code and the image comes up in the thread. Hope that helps!
I use a website www.upit.cc .. I find it to be the easiest one out there. Just go to the site, browse for the picture, click submit, then it will give you four links in grey boxes.. Just copy and paste the second one (BB Code) into the topic and post it!