I asked this question, but the images I used are rather large. Can you/how can you resize images in the question (short of downloading the image, resizing it on the computer, and re-uploading the question)?
This feature exists, it's documented in the advanced help.
There's also a quick way of resizing images that have been uploaded to Stack Exchange's servers (or any imgur server, in fact). From Resizing an image in a post?: given an image URL like