Hi Ántonia,
The page links you used will not work with the image tags. With these tags, you'll gave to use an actual image URL, ending with the file format of the uploaded pic, most often ".jpg"
You might be able to get these by right-clicking the image and selecting "copy image URL" from the context menu under most browsers, but there might also be an issue with unregistered users not seeing it if hosted on Google services.
If the hotlinking doesn't work, you can alternatively attach the images to your post under the message field.