I've created a quick tutorial showing how to post images using Imageshack. I noted that a few people were unsure how to do this, kept posting thumbnails, etc. This procedure may work for other image hosts as well, but I use Imageshack.us.
Go to imageshack.us. You have the choice of creating a free account, or not. I prefer to use my own account, so I set up a free one. Go to the media upload page, which should look like the one below.
Click on the browse button and select the file or files (you can select more than one) you want to upload. Click on the open button to close the selection window, then click on Upload.
Ensure that the image size is set to 640x480 (message board size.) You could select a higher resolution if you wanted, but under most circumstances 640x480 is pretty viewable. It's not really necessary to put things under tags, unless you want people to be able to publicly search for these photos via Imageshack. Click on Upload now when you've set the image size.
Your image(s) will now be uploaded to Imageshack.
Depending on whether you uploaded one image, or multiple images you will see two possible screens, showing your images. Click on the image if it's a single image, or right click on the image and select view image if you uploaded multiples.
You should now see a screen similar to the one below.
Click on Embed this image to expand it, then right click on the text box to the right of the word Forum, and select Copy.
You now have the code you need to post the picture in your forum topic, but there's still a bit more to do.
When you're typing your post, and you're getting ready to put your picture in - DON'T click on the insert picture button. Just right click and paste your copied code from the previous steps. It should look something like this.
Uploaded with [URL=http://imageshack.us]ImageShack.us[/URL]
We want to clean this up a bit, so delete the line where it says "Uploaded with .... to the end of that line where it says ...URL]
If you find this helpful or would like to have me change something, please feel free to comment.