I lost most of the registered LGB loco icons with the latest CS3 update, v. 2.3.1(8) too! It was because most of the icons were moved to a new "LGB" subfolder in the "lokicons" folder to make them easier, supposedly, for users to find. The result was the link to the pics was lost. I had to use the "Edit Locos" function and click on the ??? to navigate to and restore the pics. I don't use the Mainstation App, so am not sure what you need to do to get them back on your tablet. I suggest closing out the app, re-boot the CS3 (after the icons are restored), than open the Mainstation app again to see if that works.
Failing that, try out the web browser based Control app. If you do use the web browser app, set your tablet or phone to "Request Desktop Website", otherwise the browser app may not screen update properly. I found this out when I was trying anything I could think of to fix the screen update problem I was having on my iphone. Requesting the desktop website seems to have fixed the problem.