Post > New Work
Fill in required fields, and in your work text, include a rather large image
On a screen that is not as big as your image, view the work
The image is too wide to fit on the screen. It scrolls sideways. Meh.
The image should have a maximum width of 100% of the container size. (Note that the container isn't the same width as the screen.
This needs an announcement first, which is in progress.
Test on images that have width and height attributes set, just width, and just height.
This will apply to any user- (or admin-) inserted images, so it wouldn't hurt to test large images in comments or admin posts as well.