Just as a first check, make sure that you are using the correct code to display an image within a post. When typing the content of your message where you want to include an image, it should look something like this, with image tags surrounding the link to where the image is hosted:
[img]link_to_image[/img]
You also want to make sure that the image is properly hosted. You mentioned using a random image as your test image. Where was it being hosted? I suggest that you stick to images that you have uploaded somewhere yourself, rather than images that are already shown on other websites or that someone else is hosting somewhere. There are several different popular websites with free image hosting services available.
If you do not want to upload your images somewhere else for hosting, you could also try to attach them to your post if they are small enough. See the "Attachments and other options" section below where you type the content of your message if you'd like to do that instead.
