How do you...

...post pictures in posts?

I've always used the IMG tags to bracket the web address of the image, and use sites like tinypic.com to freehost the image;

Or you can upload a picture using the Attachments field, which is an option under the Message field when you reply, and then when added it can be inserted into the post using the 'insert' button which pops up after adding the image as an attachment.
Example;