Believe it was Rod that figured all this out, but it's been a while since I saw him post here. Believe he posted the how in a thread here (just too lazy to find it Wallace).

Maybe the link below will help.

viewtopic.php?f=12&t=3581

i looked at that file format a little. maybe you can hack it a little
thinking maybe you can copy an existing .phf file (like download a free .phf font)
open it up in a text editor.
then copy image data encoded in base64 and decode it
using this site.
http://www.motobit.com/util/base64-deco ... &acharset=
just copy after the 2 lines that says ...
Content-Type: image/png; charset=US-ASCII; name=Image0000.png
Content-transfer-encoding: base64
It might say something else for each imagename

and starting at the beginning of blank line after these two lines copy to the beginning of the next closing image tag (</image>)
and choose decode into binary file and name it something.png.
and open that up in Gimp and edit it to your liking
I got that far I am assuming you can just draw your own images over their images and then convert it back to base64 and paste that in to their .phf file.
and try that out.
I opened up some .phf file some smaller res files only uses/defines one image.
Other highres file files define many images so you'll have to decode and encode many of them.

I sorta created a hacked process.
try this and see if it works. This is for windows...might work in linux i don't know.

1) Have Python 2.7.11 installed.
2) Have openssl installed from here http://gnuwin32.sourceforge.net/packages/openssl.htm (I just used the first setup link. My Python script will call openssl command to decode and encode base64 the png files/data.
3) add the path to openssl to environment variables so that openssl can be called from anywhere, I added C:\Program Files (x86)\GnuWin32\bin to my environment PATH.
4) download this file and extract it to some folder the same as your free downloaded .phf file, it'll be named test.py

5) in a command prompt, run

it 'll ask you to enter a name of a .phf file (enter the full filename)
then I'll give you 2 choices 1 or 2
enter 1
1 will extract all the .png file(s) from the .phf file and dump it to the same folder.
This process decodes from base64 to png files
6) now go check out the png files and overwrite them, edit them or whatever.
7) when you're done editing png files.
run python test.py again, enter the same .phf file name
and choose choice 2
This process will now encode the png files into base64 and put them along with .phf original information into a file with the same name as the original .phf file except with a ".new" appended to the end of file name...
you can then rename this file to .phf file your your choice .

Try it out and let me know if this hack works for you.

Thanks Tin.
I did it just as you described, but when installing the PHF file.
Photofont says that the PHF file is invalid.

which plugin are you using maybe i'll try to install it myself to observe what's wrong with my output file

where did you get it? can you link me to it? i tried going to some link and it's broken from this gimpchat forum so i don't know where to get it.

the link works, i just had to sign in to drop box.
so what do i do with these files drop them in my gimp's plugin folder?
I think you linked me the wrong .zip i only see .phf files and no .exe's

i'll try and see what happens...i am determined to get this hack using existing .phf to work

I did everything except this step...how do i do it?
In Gimp I tried to look for PSPI or PhotoFont and nothing showed up so I don't know what to do next.

never mind i see it.
and got PhotoFont to start.
Now i just gotta find out why it's invalid

Is PhotoFont Free? I have the file downloaded, I just haven't installed it.