You have to have your signature picture on a host site like Photobucket or Imagestation. If you use Photobucket.com then once you've uploaded your signature you just copy and paste the link which will be the line that has [IMG]...[ /IMG] for signature under User CP here. Also, they request that signatures only be 25K or less in size here so they won't take so long to load.

As for posting regular pictures in your post it works the same, just copy and post that [IMG] line at photobucket and paste it in your post here. If you use Imagestation.com read the link I posted above in my first reply here. It explains it all.