Two ways.
1) When making a post (and not just a Quick Reply, but with the full editor), click the "Manage Attachments" button near the bottom (below the Submit/Preview buttons). You can then upload the images you want to attach. There is a chart showing the maximum sizes.
2) Find the URL to an image. Usually, you can right-click and hit properties to find this. Then, when making a post, click the button above the text window that looks like mountains and a yellow sky. It'll ask you to enter the URL to the image (I usually copy and paste) and the code will automatically be placed into your post.
Uploading a photo to Imageshack or Photobucket or other online program will most likely give you the code itself to copy and paste into your post.