Ah, you mean adding a photo here in the forum. That's easy:
You post the image to an image host like Imgur or Flickr and add the link here in your post. A full link will automatically be displayed as a link but you can also select a word like 'photo' and then click on the 'http' button to add the link.

If you want to share 3D models or a full plan you can use a site like Mediafire or use the Sweet Home 3D models page on Sourceforge .