Example. I want a crown image. I look it up on Google. I open the image to find its URL. Copy that. Then put the image tags around it and paste. Click submit and boom. Done.
Another easy way is to use photo bucket. After uploading the photo to the app, you view the picture and click the chain icon, then copy the img code. Now, in the forums, just paste the code and Tah Dah! You posted a pic!