On Windows, you can install Cropper, and there are plugins to "Send to TinyPic" or "Send to ImgUrl" . The way these plugins work, you take a snapshot of the screen or the current window, and it is auto-magically uploaded. The clipboard is populated with the short URL for the direct image.
At that point, in your SO post, you need only Ctrl+G, and then Ctrl-V, and you have an image in your post.
Very fast, very simple.
There is also a "Send to Paint.NET" plugin for Cropper if you want to doctor up the image before posting it.
Cropper and the cropper plugins are separate downloads, but both free.