www.fileinfo.com is pretty useful for identifying files that you're not sure about.
Here's their definition about .tex's:
Not the whole truth in case of Orbiter. While it's true that they are textures, the format of those files is different and Orbiter .tex files can't be opened by the same programs or games as .tex files described there.
Orbiter .tex files are containers of Direct Draw Surface (DDS) files used to store texture patches or tiles that will be applied on different spheres (surface of spherical planets, cloud layers, celestial background), or planetary rings (level1 = 1/8th, level2 = 1/12th & level3 = 1/16th of the ring).
Like TSPenguin said they are multilevel. They can contain from 1 DDS file (texture patch) inside of them for level 1, to 501 patches if they are level 8, and for levels 9+, a variable number of tiles, depending on their description contained in .bin file.
ImageMagick recognizes *.tex and a truckload of other formats.
ImageMagick doesn't recognize the .tex file format, but just .dds file format, from which the .tex file is made of.
On the command line, converting it is as easy as typing 'convert Earth.tex Earth.png', and I imagine GUIs are also available for it.
ImageMagick will convert only 1st DDS found in the .tex file this way, so just level 1 texture, or the first tile from level 9+ texture file. As such ImageMagick isn't the best program to convert these files.
To convert .tex file to a bitmap that can be edited by graphics editor you can use [ame="http://www.orbithangar.com/searchid.php?ID=4114"]Tune Your Planet v0.3[/ame].