You can't post pictures that way. You need to upload them using an imagehosting site such as ImageShack or tinypic. There's a guide [
here] which explains in detail how to upload images. Basically, you go the site, click "Browse", find the file and click OK, press the upload button, and copy the code the site provides in order to post it here.
EDIT: I just noticed your other topic. You were able to post an image in that one, just make sure to follow the uploading steps and you'll be able to do it again. The problem isn't with your computer, it's that you didn't upload an image this time, you only gave the filepath.