I can help.
There are two problems with what you're doing here. First of all, your site's mainpage is called "mainpage.html", correct? This is fine, but it's an HTML file, and not a folder. All of your images are still on the main directory of your site.
Solution : take the "mainpage/" out of your image URL.
Second of all, while linking to the .txt file is the correct way to go around the image blocking, the .txt file also has to exist. The browser can tell if there's a file called .txt that's actually an image, but it won't go looking for an image file if no such .txt exists.
Solution : Copy the image on your computer, call it "image.txt", then upload that file to your site.
Go Moon!