Unable to find .jpg file for Preview Image

Hi, I’ve followed the instructions here to try and upload a mod to Steam Workshop, but no image file comes up when I click the Browse button. I’m on a Mac and just to be safe, I’ve saved a copy of the .jpg file in each of the following locations:

\Library\Application Support\Steam\SteamApps\common\Democracy 3
\Library\Application Support\Steam\SteamApps\common\Democracy 3\Democracy 3
\Library\Application Support\Steam\SteamApps\common\Democracy 3\Democracy 3\Contents
\Library\Application Support\Steam\SteamApps\common\Democracy 3\Democracy 3\Contents\Resources
\Library\Application Support\Steam\SteamApps\common\Democracy 3\Democracy 3\Contents\MacOS
\Library\Application Support\Steam\SteamApps\common\Democracy 3\Democracy 3\Contents\Resources
\Library\Application Support\Steam\SteamApps\common\Democracy 3\Democracy 3\Contents\Resources\data
\Library\Application Support\Steam\SteamApps\common\Democracy 3\Democracy 3\Contents\Resources[mod name]

But the image file just does not appear in the ‘Select Preview Image’ box, even after I have restarted the game. Is this a Mac issue? Thanks.