It's most probably a custom. You'll have to figure out what is what in the header and program yourself something to go from the xt format to something you're comfortable with.
Here's something to get you started:
Header is 48 bytes long. Image is 128 x 272 pixels, 8bpp. Tiles are 16x8. The palette starts at 0x8830 and has a 64-byte header (that's the last row in the image). What game is this?
Wow, that's awesome, many thanks! I wouldn't mind having to write my own converter, now that I understand the header. The game is Genso Suikoden: Tsumugareshi Hyakunen no Toki. I'm a big fan of the series (shame it's dead), so I'm just poking around to see if a translation is feasible, for the heck of it.
If the format is not too complex, I may try to implement it into Rainbow, if you want: http://www.romhacking.net/utilities/1069/. But, it would be better to have some other example files, to understand what the header bytes mean.
As Dashman suggested, I also think it's a custom format, since I couldn't find any information on it. I don't know how valuable it would be for you to add it to Rainbow (cool tool by the way). If you want to implement it anyway, I'd be more than happy to help.