I'm surprised that google picasa web is not used more.
They are big, stable company. I've used them since Mar-2007, 15GB of free storage.
It's easy to upload directly to that from my Mac via iPhoto or Aperture. Same from WinTel computers.
You can directly upload also from within the web app, or using their small picasa application.
Plus, for smartphones so easy to upload from them to picasa.
Then, you can specify the size of the file being displayed easily, like this:
picture size simply by the "s800" or "s1024" or sXXX, the XXX being the pix width desired.
Test of photo at 288 pix via "https://lh4.googleusercontent.com/-8LFIEU2a4eY/U-Q3eYYiJ6I/AAAAAAAASa8/HCibQMycwTY/s288/Aug-7-2014%2520Star%2520Trails.jpg":
Test of photo at 512 pix via "https://lh4.googleusercontent.com/-8LFIEU2a4eY/U-Q3eYYiJ6I/AAAAAAAASa8/HCibQMycwTY/s512/Aug-7-2014%2520Star%2520Trails.jpg":
Test of photo at 1024pix via "https://lh4.googleusercontent.com/-8LFIEU2a4eY/U-Q3eYYiJ6I/AAAAAAAASa8/HCibQMycwTY/s1024/Aug-7-2014%2520Star%2520Trails.jpg":