there is a problem with the dropbox images due to changes in the Dropbox policies :

"Important changes to the Dropbox public folder
Dropbox accounts created after October 4, 2012 won't have a Public folder. If you'd like to quickly share files you can use a shared link. Shared links work even if the person you're sharing with doesn't have a Dropbox account.
Dropbox Basic (free) users
As of March 15, 2017 the Public folder in your Dropbox account has been converted into a standard folder. By default this folder is private to your account. This transition will occur automatically.
Here's what you need to know:
If someone visits a link to a file in your Public folder, they'll see an error page …"

that's why the direct links of dropbox isn't working anymore ..
please update your workshop files accordingliy
you have an interesting mod and I'd like to have all its files uploaded so I can play it without errors