You should be able to upload the ad image and just drag a regular text widget into the sidebar pasting the image link code in the text area. I am sure it will take a little css tweaking but it will work.
The other option if you plan to have multiple ads, or paid ads, etc. is to search for an ad management plugin.
May I make a suggestion on your website?
You really need to optimize all of your images. You have images at over 4000 pixels wide that are way too large of a file size for the web. I ended up stopping the page load because it never loaded due to all the images.
You can use something like photoshop or gimp(free) to do this. There are some online solutions too. This is a very important step I hope you consider.