Get the Pretty Links plugin – it will let you choose the slugs for your links, so you could have something like ayoungpro.com/tradeking instead of the long, scary-looking affiliate link Commission Junction gives you. So then you would upload the image to your post like any other image, and in the media popup you can choose whether to link to the image itself, nothing, or a custom URL (which is where you'd paste your Pretty Link).
If you're given a code for the image with the affiliate link included and just wondering how to insert it, you can switch to the text editor versus the visual editor in your dashboard and paste the code. When you switch back to the visual editor, you should see the image embedded in your post.
Hopefully that makes sense. Feel free to give me a yell if you need help!