First thing to remember is that if you want to upload the pictures as an attachment, the only way to do that is through clicking the post reply tab in the lower left hand corner of the page.
Simply using the quick reply box isn't going to do it.
From there its simple. You just scroll down to the additional options section and there will be a tab in the middle called manage attachments. Click that.
It should open up a box where you will have the ability to take pictures from your hardrive (via browsing) and upload them here. Unlike on some sites, this site only allows one upload at a time. So that means you have to find the picture, then click the upload button next to that. Once that's done you can upload another in the same box. You are limited to 5 attachments per post. If you have more than five then another post will be required. Also, the valid file extensions are listed for you in the upload box, so be mindful of file-type.
WAIT until the picture shows up in the upload list before continuing to upload another.
If you don't wait and it doesn't show up, that means the picture didn't go through and you'll have to do it over again. And if you wanted the pictures in certain order that messes everything up.
EDIT:
You can also edit your post(s) to include/add more attachments (up to 5). I've just edited my own post for you as an example by uploading one of my art pieces.
You can also share pictures here via hosting. That would require you use a server host like photobucket.com and other similar sites. The process is a little more lengthy though, so most people choose attachments over displaying links as images when that option is available.