1) find picture on the internet
2) right-click on the picture
3) choose Copy Image Location
4) go back to your post in PC
5) type [image] (or use the "Instant UBB Code")
6) paste the url of the picture (should end in .jpg or some other picture format)
7) type [/image]

Instructions are also in the FAQ, here:
How to include a picture in your post

Note: To keep the forums loading quickly for everyone, it is recommended that you do not exceed 35k for your image size. You can find the size by right-clicking on the picture and viewing its Properties