You'll need to get an external host that allows you to show images on different servers (generally referred to as "hot-linking"). You cannot post images directly from your hard drive, or attach them to our forum.
2 good options are:
Note: To use geocities you'll need to save your images from .jpg to .txt then post.
Once you have your host you will use the IMG tags inside the post to show where the image should be displayed.
If you need help with vB Code, please read the vB code list
Please see jetpack42's comprehensive guide How to post a picture on the forum
for detailed information.