If you're downloading the file locally you should either be able to control the name given to the local file, or be able to rename it after the fact. For example, using curl, -JO downloads the file using the remote name specified in the Content-Disposition header, while -o lets you specify a name:
$ curl -L -JO "https://www.dropbox.com/s/uex431ou02h2m2b/300x50.gif?dl=1"
...
curl: Saved to filename '300x50.gif'
$ ls
300x50.gif
$ rm 300x50.gif
$ curl -L -o "NewNameImage.gif" "https://www.dropbox.com/s/uex431ou02h2m2b/300x50.gif?dl=1"
...
$ ls
NewNameImage.gif
Alternatively, renaming it after the fact:
$ curl -L -JO "https://www.dropbox.com/s/uex431ou02h2m2b/300x50.gif?dl=1"
...
curl: Saved to filename '300x50.gif'
$ ls
300x50.gif
$ mv 300x50.gif "NewNameImage.gif"
$ ls
NewNameImage.gif