1) Find an image you like. If using Google image search, make sure you've opened the image in it's own tab as shown:

2) Right click the image and select 'Copy image address' (works in Chrome):

3) To insert into a post, first hit reply:

4) Then hit the little picture icon as shown:

5) This puts image tags into your post:

6) Put your cursor in the middle of the two tags and right click, paste the copied image link:

7) Hit post. You're done.
8) Once you become pro, you can just type [img][/img] into the quick reply box and paste in the link avoiding steps 3) to 7).