Does anyone know where can I style the image retrieved using the_content() tag?

Well not knowing how you are using it, I would suggest that you use it where ever, then look at teh generated page source and find out what it creates then use that to build your CSS selector and rule to customize it.

<img class="alignright size-medium wp-image-127" ... >

This is what the source code is showing where the image is displayed using the_content().
I have little to zero knowledge on CSS and even trying to read a bit of documentation didn't help.

I tried to put:

img#wp-image-127
{
border: solid 1px #dddddd;
background-color: #f3f3f3;
-moz-border-radius: 3px;
-khtml-border-radius: 3px;
-webkit-border-radius: 3px;
border-radius: 3px;
}

in HTML/CSS inserts, but since it's not working, I reckon it's stupid. Stuck again.
btw: the site: www.cashewcenter.com
I am talking about the image that appears on the right side of "Welcome to", which on the full About page is bordered.

a class= uses a '.' while an id= uses the '#' so try 'img.wp-image-127'

It works! Thanks alot!