[BobCratchett]

Now, new problem, I have this :

Usually (with non dev branch), I have the season poster instead of this episode thumb which fits bad. Again, did I do something wrong ?

Thanks in advance for your answers that keeps me from ripping my hair off

I've just double-checked the code to be sure, but this hasn't changed at all in the dev branch. The image used is chosen by $VAR[PosterVar]

Code:

    <variable name="PosterVar">
        <value condition="Stringcompare(ListItem.label,..)">DefaultFolderBack.png</value>
        <value condition="Container.Content(episodes) + !IsEmpty(ListItem.Art(season.poster))">$INFO[ListItem.Art(season.poster)]</value>
        <value condition="Container.Content(episodes) + !IsEmpty(ListItem.Art(tvshow.poster))">$INFO[ListItem.Art(tvshow.poster)]</value>
        <value condition="IsEmpty(ListItem.Art(poster))">$INFO[ListItem.Icon]</value>
        <value>$INFO[ListItem.Art(poster)]</value>
    </variable>

That means that either the container isn't telling Kodi that it contains episodes (which can be true for add-ons, but the type of add-on that does this can't be discussed on these forums), or the item doesn't have a season.poster, a tvshow.poster or a poster artwork associated with it, so the icon image is being used instead.

(This is true for the standard video info dialog which you appear to be using from that screenshot, not the extended info dialog)

Having said all of that, if you've been editing your menu then first try re-starting Kodi to see if it appears then. The changes to the menu editor I talked about are to fix a scenario where artwork can stop appearing within Kodi after using the menu editor.