I continue to find the MM a hard read when attempting to glean the more detailed points. Be it a resourcing issue with space, translation or technical competency there are both errors, contenting topics and incompleteness of information in the MM2010/06(English) which I propose some errata and (hopefully) clarification for. Your mileage may vary.

Ref: article Mobile Station, over to you pg16, 2nd Column
If you do not use a terminal, a Mobile Station can also be connected to the terminal connection

Propose:
If you wish to connect a 3rd Mobile Station (in addition to the 2 that can be connected directly at the front of the Central Station) and do not wish to use a terminal, a Mobile Station can also be connected to the 60173 connection.

Note: Do not try to force the 7 pin (60214 adapter) or 10 pin (direct) connector from the Mobile Station into the 9 pin socket used to connect a Terminal (60125) to a Central Station 60213/4

Ref: article Questions and Answers on Märklin Digital pg18,
60061

Comment: This Q&A rolls the subject of the power supply (60061) AND the devices it is designed for (Central Station 60213/4 and Booster 60173/4) into a single mashed up paragraph.
To be clear - the software referred to (for the "Gleise(Track) Format Processor" - GFP) is only in the Central Station and Booster devices listed - and not in the power supply. The software does not control the Power supply, however it does manage how the Central Station or Booster draws power from it - most understandably in track short situation - and as such is important to ensure that the power supply is not unduly stressed.

Ref: article Questions and answers on Märklin Digital pg18,
Transfer of Pictures to Central Station 60214

Comment: This Q&A insults it readers by being prescriptive, providing conflicting information and being incomplete.
1) Image preparation
- Any image manipulation program that can produce ".png" format files can be use
- Images may also be sourced from elsewhere - e.g. internet
2) restrictions
- The max resolution quoted as 100x36 pixels conflicts with the Central Stations own help which states max values of 128x48. Further all images preloaded conform to this with a max length of 128 OR a max height of 48. Because the image being represented may be longer or squatter most will meet the max resolution in one axis before the other.
- not mentioned , but of the utmost importance if using the USB option for image transfer, is the fact that the file name extension (the ".png") part must be in lowercase. Further - the name of the directory "icons" must also be in lower case.
3) Upload (transmitting image file to Central Station 60213/4)
- The Q&A leaves little doubt that there is only one way to transmit images, via USB!, When there are potentially 2 others.
- - The simplest is to use a LocoCard that somebody else has prepared
- - Alternatively you may connect the CS to your PC and access it with a web browser. Use the URL/Address of http://192.168.1.54/icons/ replacing the 192.168.1.54 in this example with the address of your Central Station. This will display a list of the locomotive icons currently loaded and, at the bottom of the page, provide a dialogue box to select an image direct from your PC and upload to the Central Station (No mucking around with USB sticks)

The German text reads "100x36 will take you on the safe side". They don't say it's the maximum. (16x16 probably is even safer ...)
The interesting part is left out: how do you create images with transparent background.

I don't know why, but when I think about MM I often think about Eric Arthur Blair ("Ignorance is Strength.", "1984").

In one article they had a picture of a BR 140 pulling tank cars. A close look reveals that originally the rear pantograph had been raised, but by using an image processor they swapped the raised and lowered pantographs - they did it imperfectly so the fake is easy to see.
Märklin TV revealed that there are many small fakes in the images that are not easy to see.

I am unclear what you call interesting, and why creating images with transparency has anything to do with Märklin.

I think Märklin have done extremely well to offer the functionality to add/update images on The Central Station 60213/4 and LocoCards. And, while I decry the (in)consistency of specifications, I do not for a second expect them to delve into the area of education about artistic or graphic design direction. If YOU want to use transparent images then YOU create them.

Note: Some time in the near future the statement "support for .png files" will start to imply "with animation". For now, if you want to mean that you have to state it explicitly. I do not see this as a Märklin priority.