Sometimes the moderators/admins do that kind of magic in the background when we come across large photos.
When you insert an image using the image button to insert the URL, the image tag is the letters "img" and "/img" surrounded by square brackets as the start and end of the image line to display the image.
IF once you have the image displayed using these UBB tags as they are called, you MAY edit your post ONCE and change the "img" and replace it with "thumb" and it will then display the image in a thumbnail. BUT if you try to edit the post more than once, it will screw up the code and you have to start from SCRATCH...
Much of this trouble will be lost when we move to the upgrade of the board when the site owner (John D.) is ready to go...
In the meantime, it just takes practice...
ask when you need help in this forum and someone will eventually come to your rescue...