Can’t upload or post images

When I try to upload images using the “Upload” function on the new post page, it’s not working. After I select the image and click “Upload”, the dialog box pops up asking me to enter the name, choose the size, etc. But the image doesn’t display in the box. Instead, there’s a little blue question mark indicating the image wasn’t uploaded. I tried with three different images and got the same result.

Here’s a screenshot.

Any ideas?