how to put a margin around the images of the posts

Click the picture, then click on edit image icon on the top-left corner, after that the image properties window will pop up.
Select advance setting.

Did you use horizontal and vertical space?

I think your theme style sheet is overwrite css rule.
Could you tell me your url.

Your current image tag is

<img class=”alignright size-thumbnail wp-image-103″ width=”100″ height=”100″ alt=”Imagen 2″ src=”http://visual.gi/blog/wp-content/uploads/2009/09/Imagen-2-100×100.png&#8221; title=”Imagen 2″/>

It doesn’t have

style=”margin: 10px;”

or something like this.

so, please try to put this attribute
margin:20px; put it into style in advance setting