When was the last time you tried? You need a minimum number of posts to be able to post a picture. But I think it's 10 and you're over that, so that shouldn't be the problem.
You need to upload the picture to a hosting website. I used photobucket.com and the DIS photo gallery
http://photopost.wdwinfo.com/
Once the picture is uploaded there, you have a few ways to post it.
1 - if using photobucket, just copy the link, it will already be formatted for you.
2 - other hosting sites, copy your photo link (you might need to right click on the photo and select properties), then above your post box there are some formatting options, select the yellow box that looks like a picture of mountains, paste your photo link into that box
3 - copy your photo link, paste it into your post and add
to the end of the link