see http://www.proz.com/kudoz/673167

They're also called MUPIS.

For some reason I can't copy from this page, but it describes them:
http://www.lacoctelera.com/trinamita-lab/post/2005/08/11/mup...

Cabinet & Light Box Signs

General term for a traditional "box" type sign made up of a face and back or two faces, with designs ranging from simple plastic faces to metal faces with push-thru letters. Often called lightboxes, cabinet signs can be internally illuminated, externally illuminated or non-illuminated, and are good for both interior and exterior applications.
http://www.baymark.com/TypesOfSigns.html#4

Often refered to as lighted sign boxes, lighted sign cans, illuminated sign cabinets and the like, these lighted signs are the industry standard for night time signage identification.
http://www.signsbyweb.com/outdoorlightedsigns.html

The second link above has small illustrations of different kinds. The ones I see in Madrid are usually the "one pole" type.

Pictures of "Mupis":

http://images.google.es/images?hl=en&q=mupis&um=1&ie=UTF-8&s...

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Note added at 21 hrs (2007-12-20 05:29:30 GMT)
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Here is another picture of what MUPIS are.

http://www.redext.com/pag/impursa/mupis.html

And another definition:

Las mupis --***publicidad insertada en paneles urbanos***-- se han extendido paulatinamente en la última década. Son pocas las avenidas o plazas donde no hay una de estas estructuras, casi siempre con formas originales o acompañando a otros elementos, como los contenedores de pilas.
http://www.elperiodicodearagon.com/noticias/noticia.asp?pkid...

Another:
unos mupis (elemento de mobiliario urbano que consisten en una caja iluminada o no, colocada verticalmente sobre un pie que habitualmente exhibe carteles publicitarios)...
http://www.diariodelaltoaragon.es/noticias/detalle.php?id=25...

As you can see, these have advertising INSERTED in a BOX, which can be illuminated or not. If you compare these definitions to the ones above in English (not to mention compare the photos), you can see they're the same thing.

MUPIS are differentiated from "vallas publicitarias" and other types of outdoor advertising:

A la categoría de Publicidad exterior se pueden presentar vallas, mupis, carteles, publicidad en autobuses, etc.
http://www.premiosalce.es/2007/69/bases-para-profesionales

Out-of-home media or advertising lumps all of these things together:

Static billboards, one of the oldest forms of advertising, is the largest out-of-home medium. The tactic is expected to rise 6.8% this year to $4.13 billion and grow at a compound annual growth rate of 6.2% through 2010. Digital technology is expected to fuel the growth with a greater rollout of digital billboards, according to the forecast. Transit displays (think ads on subways, airports and buses) are projected to grow 8% to $1.34 billion this year jump 7.4% on a compound annual basis through 2010. Spending on street furniture (think bus shelters, trash cans and park benches) is projected to rise 4.3% to $947 million this year and grow 3.5% annually through 2010, the forecast said.
http://promomagazine.com/research/other/ofh_media_091106/

A list of out-of-home media categories:

Out-of-Home Media Categories
--- Place-Based ---
Airport
College Campus/School
Convenience Store
Hotel
Indoor
In-Flight
In-Store
Movie/Theater
Shopping Mall
Sports/Fitness/Leisure
Stadium/Arena/Sports Team
Truckside/Truckstop
Event Marketing
Other Specialized Media

---Traditional Outdoor ---
Aerials/Inflatables
Bus Bench
Bus Shelter
Digital Media
Mobile
Outdoor
Sampling/Street Teams
Taxi
Telephone Kiosk
Transit
Vehicle Wraps
http://www.srds.com/frontMatter/ips/out-home/index.html

The last link shows that out-of-home media can even include in-flight advertising (!), so this term is way too broad for what MUPIS are.

(electronic) billboards