For image sharing I use imgur.
Everytime I upload photos it tells me there is a connection problem and that I have to go back to reconnect... It's the only website that tells me this, I can upload on Facebook and 500PX.
How many photos are you trying to upload at a time? I get this problem too, usually when I try to upload more than 5 photos at a time and if Flickr is having a bad day.
Iv uploaded 10+ at a time on Flickr. It takes a while though. Especially with my connection.
I uploaded 274 photos all at once and had no issues with FlickrI find that if you upload more than about 5 at a time it can crash, but we'll see if it's any better with my new fibre broadband.