I've already was able to use content in my project,but now this error keeps appearing everytime.In the pictures i used the "smiley" for example,but i've tried with other images and have the same error,except for bola,shuttle,earth and stars.
Your image files are in a Imagens subdirectory. You need to include that in the path (Imagens/smiley).
I recommend you check the inner exception whenever you get a ContentLoadException. It will contain details and give you the path where it is looking. How it comes to that path and how MG copies xnb files there when you build your project is explained in the docs in case you want to understand this better (it helps if you run into issues too): http://www.monogame.net/documentation/?page=Using_The_Pipeline_Tool (check under Building Content)
Unrelated, but relevant: posting images is not very helpful. Text is usually way better to provide information about exceptions. Nice explanation: http://idownvotedbecau.se/imageofanexception/
Thanks for the help,i'm a begginer.Now it's working.
In the MonoGame Pipeline Tool (MGPT), you can copy the asset's path in the properties on the bottom left to avoid these mistakes And be sure the output of MGPT matches your app's content folder.