In Firefox: just right click and choose "copy image location". then in a post, click the image icon above the posting area (looks like a sun above a mountain) and paste into the pop up box.
In IE, the easiest way to insert a pic is to right click on an image from the net, and choose save image to save it to your computer...then when posting, click the Attachments box under the posting area and upload that image from there.
Note: When posting a question (aka "starting a thread") you always see the Attachments option under the window where you type...but when answering a question, you need to click the "Go Advanced" button under the window to see the Attachments option.
Here is a thread where you can practice.