Images are easy to upload, by attaching files, or drag and drop. However, the size can't be bigger than 512KB. 
You have to scale the image size down - I may put up a quick screencast.

Free editors:

https://photoshop.adobe.com/resize (have to do a survey at the end, but by far the easiest to use)

https://pixlr.com/pricing/ (have to sign up, but they have a free plan)

Hope this helps!