The size of the image file does not matter. This is caused by wrong URL.
The little icon is added by your browser due to an error, it means "Broken image" - browser cannot display the resource that you referred to as an image URL.
You are trying to use this URL for your image:
which, despite the fact that it ends with .jpg, leads to a webpage, no an image file (try visiting it yourself now - it displays a Dropbox page with that image and some other elements like text, links, other images and buttons). A webpage may not be displayed as an image, hence a broken image icon is displayed.
What you need is to find the real direct URL of an image as a file, not a link to the Dropbox page. Make sure your image is in the Public folder of your Dropbox, and in the Dropbox web interface click the file in the Public folder and then click 'Copy public link' in the toolbar:
Now, that one URL can be used to embed your image into your category description.