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Is it possible to add icons like this for ratings:  and is it also possible to change the standard white square media icons?

Would like some options for these icons:  Have anyone here change or done anything like this?

A Noob's Guide to other Noobs
I am a total noob in skinning and programming but I managed to add MPAA rating flag. See PG-13 below. So yes its is possible, not easy but possible.
About replacing the Flags with other options. This is also possible. You just have to replace the code that displays a particular Flag with one of your choice. For this skin, and most likely many other skins the codes for flags are in the "Includes.XML" file. This file is in the xml folder, the xml folder is located in the Addon folder (the folder is skin.estuary.modv2). Where the KODI addons folder is located depends on your system, you check with KODI wiki if you are not sure.
What code you need.

Code:

<texture> some code here <texture/>
<visible> some code here </visible>

The code that goes between these two elements are the main code that display your flags. You can find this code in other skins that have what you need. In your screenshot,the code displaying the ratings icons can be found in the includes.xml of that skin whose screenshot you took.
Example:

Code:


<include content="MediaFlag">
                    <param name="texture" value="$PARAM[audiocodec_var]" />
                    <param name="visible" value="!String.IsEmpty($PARAM[infolabel_prefix]ListItem.AudioCodec)" />
                    <param name="visible2" value="!Skin.HasSetting(show_mediaaudiocodecflag)" />
                    <param name="top" value="5" />
</include>


Explanation:
Our <texture> code is contained here <param name="texture" value="$PARAM[audiocodec_var]" /> this code says fetch the icon for the AudioCodec of the video, AAC in my case, Dolby digital in your case.
Our <visible> code is contained here <param name="visible" value="!String.IsEmpty($PARAM[infolabel_prefix]ListItem.AudioCodec)" /> this code says display the icon if the AudioCodec is not blank. If your video has no codec (blank), this code avoids the skin showing an empty rectangle where the icon is supposed to be.
The codes inside <texture> and <visible> are the most important.
Now this <param name="visible2" value="!Skin.HasSetting(show_mediaaudiocodecflag)" /> says, show the icon if the user has changed the settings to show MediaAudioCodecFlag, if not, do not show the icon.

Where to put the code:
Once you find the code you need, it is time to find where the code goes in Estuary MOD V2.
To avoid your skin from breaking, try to maintain the original design as much as possible. As you can see, my MPAA rating flag/icon fits almost perfectly with other flags. This is because I maintained the design.
All Media Flag Icons in Estuary MOD V2 go under the <include name="MediaFlags"> this starts on line 725 (use Notepad++ or other text editor with line numbering).
The individual flags, starting with the studio logo begin just below <control type="group"> this is on line 835. 
You can replace any of the icons and insert your own, by editing the code here. To replace the AudioCodec icon for example, replace this code with the code you extracted from another skin. Paste only the two main codes first (texture and visible) and see how that goes. In most cases, you wont need the rest of the code such as width, height etc. The size of the icon is already taken care of by the <control type="group"> . If the icons do not show as expected, you can add the rest of the code that came with it.
To add a new icon, paste its code within the <control type="group"> 

Code:

    
<control type="group"> ​​​​​​
        to maintain the design
        your texture and visible codes goes anywhere here. 
        Before other icons, after other icons or between them
</control>

If Addon is required:
If the value under <visible> requires an addon, install that addon first. You will know an addon is required if you see the code has an addon name in it.
something like "HasAddon..."then goes ahead to give the addon name. In the example below Studio Icons - Coloured is required. The code may sometimes not say "HasAddon" but the addon name must be mentioned if its required.

Code:

<param name="visible" value="String.IsEqual(Skin.String(studiologos.path),resource://resource.images.studios.coloured/) + System.HasAddon(resource.images.studios.coloured)" />

Disclaimer!
I am a noob in this. My guide my be entirely wrong or misleading. I am just trying to help because it is important we help each other especially now that @Guilouz is unavailable (We really appreciate his work, we hope he will be back soon to update this amazing skin). This is the little knowledge I have gathered from editing this skin.