Is there any simple way of changing the dimensions of a picture when adding it to a post? (Other than downloading and change it in an editor.) Sometimes the default size of the image I find is not good for the readability of what I am writing.
//Unrelated, is meta the right place for this type of questions?
Yes. If you upload the image using the image dialog so it's hosted by imgur, you can change the name of the image to display resized ones that are automatically generated on upload:
l at the end of the image name for a 640 pixels in width large image,m for a 320 pixels wide medium image,t for a 160 pixels wide tiny image,s for a small 90x90 pixels thumbnailMind that these file name suffixes are case sensitive!
For example, https://i.sstatic.net/sI5O0.jpg is a fairly large image. By adding those letters to its file name, we can get:
640 px wide sI5O0l.jpg (large):
320 px wide sI5O0m.jpg (medium):
160 px wide sI5O0t.jpg (tiny):
90x90 px sI5O0s.jpg (small thumbnail):
