To download an image file in a Ruby application, you can use the standard library open-uri method to access a URI as if it were a file. However, this approach has its limitations and potential issues such as memory inefficiency, leaving users vulnerable to remote code execution, and not allowing for redirects or file size control. To overcome these issues, alternative methods like Down can be used to safely and efficiently download files. The Down gem provides an improved way to handle downloads by returning a Tempfile, limiting redirects, and controlling file sizes. With the Down gem, you can easily restrict file size, stream files, and even change the back-end of the gem from open-uri to other options like HTTP.rb or Wget.