It seems that you can achieve txd (RenderWare texture dictionary) to png (Portable Networks Graphics) conversion using the Magic.TXD editor for _GTA. As far as we know, this is the most popular (and perhaps also only) way to convert txd to png files.
Convert RenderWare texture dictionary to Portable Network Graphic.
We found one software available in the database for this conversion.
Microsoft Windows software - convert txd to png on Windows
An editor for RenderWare Texture Dicitionaries
Convert from txd file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.