First you need to upload your picture to the internet. I use Photobucket, but there are other good sites out there.
Next, you need to resize the photo to meet disboard size limits (the website that you use can help you resize).
Finally, you insert the picture into your post. That requires you to copy the URL for your picture (Photobucket provides the address, and I am sure the same is true for other hosting sites). Then, in the body of your post, click on the icon that looks like a mountain range (next to the "insert e-mail link" icon). The tool will prompt you to provide the URL of the image, which you simply paste into the box.
Good luck.