Assuming Thunar is using wget to download contents, it should follow proxy configurations of ~/.wgetrc.

# You can set the default proxies for Wget to use for http, https, and ftp.
# They will override the value in the environment.
https_proxy = http://proxy.yoyodyne.com:18023/
http_proxy = http://proxy.yoyodyne.com:18023/
ftp_proxy = http://proxy.yoyodyne.com:18023/

1. Set ~/.wgetrc to use Tor proxy.
2. Try to download images (by drag and drop) from Tor hidden services.
3. HTTP Error.
   Cannot resolve hostname (somehostname.onion).

Using Thunar 1.6.3