Adding images via ImageShack is really cool I use ImageShack a lot, the best part is that it creates thumbnails for big images & you can embed those thumbnails with link to the original size, all this with you doing nothing extra. Here's how:
In the quick reply box you will see the uploader. Browse to the image & click 'host it!'
It will then open a new tab/window that will look like this:
From here all you need to do is copy the code, it will automatically create the code for adding images into a forum. (refer the screen shot, highlighted)
Paste the code in the reply text box & you're done.
You can also create an account on Imageshack, though it's not a compulsion for being able to use the service. If you use the imageshack site, you have options like auto-resizing the image to specific sizes as well:
Select the reso from the drop-down 7 check the 'Resize image' box -
if you want to embed the full sized image without thumbnails, then there are 2 ways:
1. Use the 'Hotlink for forums' code
2.Copy the 'Direct link to image' & paste between [img][/img] tags.