The images don't appear when the button is pressed

The image you are trying to print is using a relative url. To have it show up on the print page, you’ll need to use a full url. So for example:

Bad:

/wp-content/uploads/2015/06/guiamandalas-1.png

Good:

https://www.guiamandalas.com.ar/wp-content/uploads/2015/06/guiamandalas-1.png

Hi Baden! Can you tell me how and where can I change that? I will be very grateful

how are you inserting the images in the first place?
you should be able to edit the page, view it using the Text Tab (next to the visual tab in the editor window) and make sure the images are using full urls.

by default WordPress inserts images using full urls. it might be that you have a plugin/theme that forces relative urls.

It looks like I’m using the correct way to insert images:

<a href="https://www.guiamandalas.com.ar/wp-content/uploads/2015/06/guiamandalas-1.png"><img class="alignnone size-full wp-image-40" src="https://www.guiamandalas.com.ar/wp-content/uploads/2015/06/guiamandalas-1.png" alt="Mandala para colorear" width="600" height="600" /></a></p>
Maybe the error is in how I use the shortcode. I don’t know much about coding so I put the target in alignone: [print-me target=".alignnone"]
Is that a correct way or I have to change it?
P.S: the shortcode is before the code of the image

Well, that looks correct, however the output on your page is:

<img class="alignnone size-full wp-image-40" src="/wp-content/uploads/2015/06/guiamandalas-1.png" alt="Mandala para colorear" width="600" height="600" data-jpibfi-indexer="0">

So, quick question: Where is that Pin it button coming from?

It’s from a plugin. I now deactivated and leave it that way but the problem continues. What can I do?

you might try to do basic troubleshooting.
1. disable/enable plugins one-by-one
2. change to a standard wordpress twenty-something theme, to see if it’s theme related

There must be SOMETHING causing your urls to be relative.

We see you still do not have this issue resolved.
Have you tried the basic troubleshooting steps we listed above?