I've just tried what you explained and come up with nothing,
if i right click an image 'copy image address' is not there
(Note: the following is PC specific, would someone like to add the Mac info on how to do the same job?)
Are you clicking on an image that's on a website?
If I go back to an earlier post in this thread and right click on the photo of handsome Jethro, I see this menu:

You can see that 'copy image address' is one of the options. Clicking this will copy the internet address of the picture.
The img (

) option on this forum only allows you to link to pictures stored on the internet. That's why when you click on

you then have to enter the image address between the [img] tags.
What you can't do on this forum is to upload a picture directly from your PC.
If you want to use a picture that is stored on your PC (and not already on the 'net) you will have to upload the picture to a webspace first. You can either upload to the personal webspace that your internet service provider will have provided for you (most of them give you free space) or you can use a website like (I think) photobucket.
Once the image is stored on the internet you can pick up its address and use it as described above.
GC