TMX

TMX is an image format used in Persona 3 and Persona 4. It is used in almost all Atlus-developed titles to this day. TMX can range from 16 colors to 256 colors with alpha transparency. An official tool for creating TMX files from TGA files was leaked in the US version of SMT: Nocturne.

Persona 3/4
TMX is used in these titles for character bustups, HUDs, and logos. Sometimes these files are wrapped in SPR containers and/or PAC/BIN archives.

Amicitia
Featured article: Amicitia (program) Amicitia is TGE's all-in-one editor for Persona formats. It features texture conversion from PNG (for both TMX and RMD). You can get it here.

TMX Explorer
TMX Explorer.exe was created by Blyss at XeNTaX in 2010 for viewing TMX files and extracting them as PNGs with transparency intact. To use it, just run the EXE in a folder with TMX images. The included zlib1.dll must be in the same directory in order to run. The tool can be found here.

BE0605.exe
BE0605.exe is Atlus's official TMX creation tool. It was found on the North American SMT: Nocturne disk, presumably used for localization. You can open TGA images and save them as TMX using this program to convert to TMX. You can also open TMX files in this program to view them and edit their palette/bpp. However, there are random glitches with this program where the TMX won't load properly. It is recommended to use either Amicitia (for TMX creation) or TMX Explorer (for viewing/extracting) instead, if possible. You can download it here.