Depends on where you want to post. In the GALLERY area you post in the correct topic area or in your MEMBER GALLERY area. You must be logged in to the site to do this. In the upper right hand corner are the options and they are pretty self explanitory. (See image below.)
If you want to post / display an image in the forums area you need to have it available elsewhere (i.e. a personal website). You then create a link to it in the body of the forum post. TIP - Having your profile setup to use the WYSIWYG editor makes it very easy to do.