[member="Mit Tuxaire"]
Copy the URL.
Above the text box after you click on it will be 2 lines of icons. You'll see things like the bold icon, this is the line you want. A bit over, about mid line. Is a icon that looks like a picture of a landscape (If you've that much trouble finding it, you can just hover your mouse over and it will say "image").
Click it and paste the URL.
Click OK, and the picture will appear in the text box.
Another issue as said before is some sites won't allow you to link their images to other sites, or have limits. If you really want that image. Use a site called Imgur. Make an account and you can upload images to it either from a url or your computer to it, and link it from there. The images rarely break. Its a top tier image storage site. Reddit was the reason it was made. I use it to store avatar images, factory submission images and the like. Keeps everything in the same place instead of having an image break and probably being lost because you can't remember where you got it from.
Any other questions or if you need help just ask.