I know it's popular but I have had nothing but problems with steemimg.com.
Half the time the images don't load, then they didn't show up as they moved all of their files to a bigger server. I just quit using it and postimage.io was so much simpler. I know steemimg is steemit user and I would love to support it for that reason but I had too many problems with it.

I know the image loading feature has changed and you're supposed to be able to drag and drop images into the editor, but again I have seen that work and not work multiple times from different people on different computers.

Postimage.org has worked for me and those I have introduced to Steemit every time so far.