That's not a JPEG, that's a Zip file.
You'll need an App capable of decompressing the Zip file to get to the image.
At this point you only get Dropbox as an option because it's the only App you have that can handle a Zip file.
There are Zip opening apps in the App Store. Try WinZip its free.
https://itunes.apple.com/us/app/winzip-leading-zip-unzip-cloud/id500637987?mt=8
Once you have winZip installed, try downloading the picture again. You should get an extra "Open In" option in Safari.
Once the Zipped archive is open in Winzip or any other Zip app, you should get a list of files inside you can view directly in WinZip, or you can copy them to your camera roll using the icon