Please don't talk down to me like I'm a moron just as Deon has done. I am a professional developer working with designers writing training applications for a fortune 500 company for a living. I know what bmps, jpgs and pngs are. I also know that bitmaps have headers that tell the program what format the data contained is. What I find difficult is:
1. The import procedures used by Toady apparently doesn't read standard BMP file headers and makes assumptions that your file is in 24-bit color depth.
2. That tiles are separated into 2 or more files when all the tiles could easily be placed in one file for easy import and user management.
3. The wiki, where common folks would look for tilesets, has PNG files for download as the tilesets (which DF doesn't import apparently and absolutely bombs by not displaying anything, error or otherwise.)
4. The tilesets don't all have preview shots so the common user can see them in action
5. The developer adds support for caves and islands instead of improving the already lackluster interface
6. Using the client in ASCII mode. You can't tell what the hell is going on with "D"s running around and "+" like symbols used for doors. It doesn't even look like a door.
And I guess this is why they don't bring in the 'consumer-oriented specialists' during the alpha, huh? I don't think this is going to be anything like a training program. Given that this entire thing hasn't made 1.0 yet, and there's a clearly defined point in which the user interface is to be improved that quite clearly hasn't been started yet, your personal and learned industry standards aren't going to be applicable.
Gamerman has given pretty much all the instruction that's really required of tileset installation, but if you were looking for a list response, I may have some answers.
1. Generally speaking, the game will be meant to be used with it's primary tileset. Other tilesets will likely be made by those who know how to install them, and thus should be made in the proper depth. Those who wish to install said tilesets should need to do little more than copy and paste, assuming the creator has not written an automatic installer, as several already have. Modding in general is rarely supported to such an extent as to be counted in release expectations, and thus such methods have been relied upon quite frequently. Remember, tilesets are not inherently part of the game.
2. Also so that one could mix and match their preference for representation, alongside access benefits and the capacity to 'mix and match' preferred tilesets for each type. Eventually, for optimal modification properties, the files would be split into further seperate representations of each individual entity and object. Said files would be placed in a folder, as they are now, to allow for instant exchange, thus rendering fragmentation issues moot.
3. Oddly, my download of the DR tileset showed all files as .bmp files. If you have found another file that causes issues, it would be best to lodge a report with the file's creator, as the wiki's nature does not allow for the issue being resolved by any other than one in possession of the full work.
4. Again, if you find such a thing, please contact the uploader of said work to pursue a solution.
5. The restoration of functions removed by the very recent programming issues does, in fact, take precedence over breaking one's schedule and seriously undermining the planning and programming proccesses to begin applying polish, which in the long run would only lengthen the development time of the program. The Presentation Arc, listed in the development page of the main website, will be taken care of in it's due time, certainly before official release, but completing it before such time would only cause further delays to maintain the interface while making further alterations and additions to the gameplay itself.
6. ASCII is not a 'mode'. The primary display of the game is made to create the feel of older games played in such methods. If you wish to alter it, persue use of the many third-party visual aids. If you wish to lodge a complaint, please do so in the Suggestions Forum, but first you may wish to read the many other claims to said complaint.
I tried to make sure I got the whole point across without dumbing it down, as I felt you might not appreciate it after having to dumb things down for training programs all day.