There are two ways to show pictures in your posts.
1) If you already have pictures on an image hosting site, you can attempt to link to them directly. Copy the URL of the image file, click on the "insert image" icon and paste the url into that. Some image hosts don't appear to allow offsite linking in this manner, such as Picasaweb(google) or Yahoo Images. Others, like photobucket.com have no problems. Using this method makes the pics appear in full size directly in the post.
2) If you only have the images on your PC, you can click on the "Additional Options" link below the message textbox. You'll then see a form box with Attach:
beside it. Use the browse button to find the file on your PC, then click OK. You can also click the (more attachments) link if you need to upload more than 1 image (up to 4). Using this method creates thumbnails of your images that can be clicked to open to full size.
If your image is too big to fit on the screen, you can resize it for free using http://www.shrinkpictures.com
Choose size: 760 and quality: best
Then be patient. It has to upload your large picture and resize it. When finished, you'll be taken to a page with your newly resized
image and a link to download it back to your PC. Choose a filename and save it, then come back to the forum and upload your resized image!