Lou's pipped me to the post with the St. George's one.

The Ethiopian flag is one of the oldest in the world, although apparently yellow used to be the top-most colour at one time. The Lion of Judah symbol is the 'imperial' one (so I guess it might appear on the Imperial Guard's flags), while St. George appears on others, hence Lou's photo.
The 3 pennant style is apparently 19th Century, but might be a nice way of distinguishing 'tribal levies', especially as some of the leaders were both old and conservative. More accurately perhaps a plain 'national flag' would be easy to make in quantity, as Cuprum's picture shows.
There is mention of a flag with white top and bottom and red centre in some articles that reference Herbert Vivian's visit in 1900, where he saw one flying from a border guard post. Sometimes the Ethiopian Cross is mentioned too. Medals issued by Emperor Menelik to British soldiers also had a white-red-white ribbon and the medal itself featured the aforesaid crosses. Best guess from me is that these colours represent the Church of Ethiopia, which had the Emperor as its spiritual head. In the photos Lou provided, you can see one to the right in the bottom photo.
I would imagine that each tribe/region had its own type though (and indeed the white and red one might be one of them), but the green-yellow-red one is the national one.
The second photo also shows bi-parti flags, if it's any help yellow, green, red or blue would combine with white on these, all having significance to Ethiopians in a variety of ways. My best guess is that these were personal standards of tribes/chieftains.
High-ranking Ethiopians also had a monogram, usually using Amharic abbreviations of their name:
http://www.angelfire.com/ny/ethiocrown/flags.html