I've never used Photoshop but it should have the option to reduce the color depth. Reduce to 256 Colors then Save As in BMP format. M$ Paint has the option to Save As in 256 Colors. The best method for FS textures is to use the Extended 256 Color option from DXTBmp.

Dogwaffle
"Martin Wright has created a collection of plugins for Project Dogwaffle and PD Pro. The plugins can import and export image formats commonly used by simulation games, such as flight simulation and train simulation games. Typically, the formats are based on an extended BMP 2000 format, some in 8-bit, 16-bit and other peculiar formats. Some use Alpha, some don't.  Undoubtedly, if you're into making mods for your Flightsim game or other games you'll find this plugin collection very useful when painting with Dogwaffle."