How to change a picture loko in menu?

Better you do this with Hand manuell.

..***Steam\SteamApps\common\railworks\Assets\Kuju\RailSimulator\RailVehicles\Electric\BR101\Default\Engine\LocoInformation\image.png

..
The image.png File is 256x128 pixel Screenshot.
Replace this with your PNG File.
The Name must exact "image.png" and 256x128 pixel.

1 Minute Work from Hand.

g'ice

Hi Dallas,
Or you could open the scenario with the scenario editor (not with RW-tools) and move the Br-218 a little bit forward and backward. Then save the scenario. Next time you start Train Simulator the preview picture should be the one of the BR 218.
(Sorry for my epic English )

kurze Übersetzung:
Er soll die Lok im Szenario-Editor ein wenig bewegen, um das richtige Vorschaubild zu bekommen.

ice:
Dallas wollte nicht das Bild tauschen, sondern wollte die benutzte Lok korrekt oben angezeigt bekommen. Er tauschte wohl die 101 gegen eine 218 aus, aber die Liste aktualisiert nicht das benutzte Rollmaterial.

Dallas:
Please ignore ice's hint of swapping the pictures. It seems that he misunderstood your question.

Your issue can maybe be solved by emptying the cache. This should delete the SDBCaches, which store information of all used scenarios. But I'm not sure if this helps.
The hint of Thalvik maybe can be a solution, too.