To add a photo to your post directly from your computer, click the 'Add Attachment' button, upload the file you want, then click 'insert'. If adding more than one photo, upload all, then simply click done - the images will be added after you click 'create post' - this method adds the images below each other. If using the 'Quick Reply', simply create your post - then edit it to add images afterwards.
The image size will appear fairly small in the post but it can be clicked on for a larger view.
Images can also be added by clicking the image box and adding the url of the image.