I’m not seeing this behaviour myself.
I too have lots of images using underscores - and others without. Not seeing any difference with regards to passing the validation. Same old ‘Failed to get a proxied URL for the image.’ error. Hit and miss as ever.
It a shame there’s been no recent response from the Twitter devs. The temperamental nature of the problem means if I can’t use with confidence - can’t really use at all.
That being said - I do seem to be having more luck with images without underscores… still not 100% though.
this issue has not been fixed, at least not for me.
weBINDD
#105
Same problem here with validated site. Example URL --> http://www.socialmediaycontenidos.com/50-terminos-imprescindibles-marketing-online-bien-explicados-1
Sometimes it works, sometimes it doesn’t.
Any update on this?
Thanks 
ccarfi
#107
I was getting the same issue. I changed the image size on my image to 120x120 and it worked perfectly the first time. YMMV.
Your images are below 120x120, just like ours.
https://dev.twitter.com/docs/cards/types/summary-card says that’s ok, as long as they’re above 60x60, but that is currently not true.
Same problem occurring here, even with underscores removed/120x120 image (http://www.tonyrobbins.com/products/career-help/mastering-influence/)
Trying with live webcam-images, seems to constantly work/not work … Images are updated every minute @ http://compuccino.tv
artegic
#112
Hi, we are having the same issue, validator sometimes says everything is ok, other times says Failed to get a proxied URL for the image.When tweeting the content everything appears ok but the image. Sample page http://alcantarastone.com/index.php/projects/transport-and-infrastructures/puerta-del-sol-communter-train-station
Robots are ok, tried changing image to another one with a shorter url, same problem. Any ideas of whats going on?
kirlink
#114
In case anyone is in the same scenario I found myself in, it turned out the failure of images to load in the Twitter card previews had to do with HTTP headers. It was confusing at first because sometimes the Card validator would green light all fields, other times display the “Failed to get a proxied URL” message. But either way no image would load.
Finally realized that one of the directories from which our images (used in the Cards tests) were being served had specific noindex,nofollow robots headers being sent out. Was to prevent random non-image files from being indexed by Google et al. Took some testing to eventually realize Twitter’s system was respecting these headers as well. Disabled these noindex headers on the images directory and all is well.
This tool is helpful to check your headers:
http://web-sniffer.net/
Hope this helps someone.
Me too.
What I’d like to know is if this is just a problem with the validator or not.
We are still having problems with this issue. it seems to be working for Tweets coming from our blog (different server) but main site its not working. Latest example image is this one http://res.hoteltravel.com/germany/frankfurt/articleimages/boutique_hotels_museumsuferfest_1.jpg
Seems image specific, only happens to certain images.
This image happens 100% down, but others work fine.
Erro: Failed to get a proxied URL for the image.
Link: http://minus.com/lqZTZ6X1rjZw6
Used: https://dev.twitter.com/docs/cards/validation/validator
Also how can we whitelist all wildcard subdomains for profiles on Minus? Example is http://john.minus.com
Thanks