When I upload a screenshot I've taken of a Mathematica Notebook on my Retina MacBook Pro, the resulting image is HUGE. How can I prevent this and get images that appear at a reasonable size? Can I get images that are scaled appropriately for Retina and non-Retina machines (currently all SE images on my machine are "blurry").
There are two ways I can recommend:
Do not take a screenshot. Instead, select the graphics, right click, then choose Save As ... and save it as PNG. This will give you a standard (non-retina) resolution version. It will have a reasonable size, but it will look blurry on a retina screen.
If you would like to include a retina-resolution figure, but display it at standard size (not double size), you can do this: take a screenshot using Command-Shift-4. When you take the screenshot, OS X will display the width of the region you are clipping. Remember this number.
Next, upload the double-size image as usual. Get the image URL from the end of the post where it gets inserted, and delete the line that looks like
![enter image description here]. Instead, write
<img src="url" width="360">where
urlshould be replaced by the actual image URL and
360should be replaced by the width you noted earlier.
No. 2. from above is what I usually do. Take a look for example at the MarkDown source of this post.
Update: I posted a feature request on Meta StackOverflow to make it easier to include high resolution images.