First you have to get an account with an image hosting site. I use Photobucket--it's free and quite good.
Image hosting, free photo sharing & video sharing at Photobucket
When you establish an account you can download images from your "My Pictures" file on your computer--I believe it's the default search target. Once in your photobucket account, you simply run your cursor over the saved image and a list of choices pops up. Select "direct link" and left click to acquire the http link. Alternatively, you can right click on the image and select "copy image location." Either way, you then go to your RF "Reply to Thread" page and select where you want to insert the image. Then go up to the "insert Image" icon (mountain with Sun) and paste the url address you saved from photobucket, and there you go.