There are at least two ways:
Right click on the image and select "save as". Now you have the image in your computer, and now all you have to do is to upload it as an attached file. You'll find the button for uploading images when you're posting a reply in the thread, if you click the "new reply" button (not doing a Quick reply) This will create a thumbnail that leads to the larger image.
Another way is to right click on the image and select "properties". You'll see, among other things, the image's URL. Highlight the URL, right click on the selected text and click "copy". Now come to the boards and paste the URl between image codes; image codes are (img)(/img), only with square brackets -> [ ] instead of round ones (so it should be something similar to (img)imageurl(/img) , but again with square brackets) This will load the image directly in the post.