When you type a message, at the bottom there is a place that says "Select Image file to attach". Click browse and select your pic. It will insert it in your post. The only drawback is that you can only insert one pic per post, so you have to post every time you want to add a new pic, and insert some text, like "picture 1", "picture 2" for example.
Otherwise you can do as suggested and host them elsewhere and just link to them using "img" tags...there is a button at the top when you type a message, it says "Img", that will create the tags in square brackets. You insert your image link between the sets of square brackets. The preset size is 150, you can change that in the tags.
DO NOT use the "Select file to attach" option, that is for attaching files, not pictures.