Is there a way to auto-upload photos to Flickr on Mac?
Solution 1:
For an OS-independent solution, the following IFTTT recipe looks like it will do what you need.
Upload to dropbox - push through to flickr
In a nutshell, the recipe monitors a specific folder in Dropbox (the recipe uses "/Flickr", but you can change that) and uses the "Upload public photo from URL" action on the Flickr channel to upload the image.
There are a number of recipes that use Dropbox and Flickr, so even if this one isn't exact (and you don't want to customize) you should be able to find one you want. Here's another possiblity: Move Dropbox photos from Camera Uploads folder to flickr gallery.
Solution 2:
Update: I found a new solution. It's an app called UPPR. It is currently $2.99USD on the Mac App Store. The app is simple and great, I use it now to monitor my OneDrive (switched from DropBox) pictures folder. It passively monitors a folder and uploads anything in it to Flickr. Super easy. You can set tags and privacy for all uploads.