It's possible that the image you are trying to enter has a bad URL. If you're getting it off of google images then you need to go all the way to the original image and use the URL. If it's still not working try changing the size as I find that also helps.
Search Metagross on Google Images.
Click on the image you want, it'll then go to this image: http://i.imgur.com/CgRXR1C.png?2
Click on View Image.
Then it will lead you to the image with the correct URL. If you want a different size then go back to image one, and click on the button beside View Image for different sizes.
Also if I could see the link for the Metagross pic you want that's be great.