There's also one for GIMP.

Just for information for the next time:
u have almost always some empty space between the file in a normal Psx cd.
I make u an example with the 2 tims u found in Chrono Trigger.
Open your image with CDMage and take a look at the LBA (start sector number) of your files.
SQEA.TIM LBA 5548
ROGO.TIM LBA 5579
Each data sector has 2352 byte of data (not the XA/STR data) so if u take the LBA of the second file - LBA of first file and multiply for 2352, u get the actual space for the first file.
(5579-5548)*2352= 72912 bytes
but if u check the file size of SQEA.TIM, it's just 61984 bytes
Means u have 72912-61984=10928 empty bytes between the 2 files that u could use.
for ROGO.TIM u have to go search for the next file on the cd (SUBMAP.DAT in the folder DATA and LBA 5618) > (5618-5579)*2352 = 91728 > 91728-77344= 14384 empty bytes.
U can expand all kinds of file in this place just not XA/STR files.
In some cases u need maybe to correct the file size in the TOC of the cd and in some other cases, this is not working because the game it's using a mixed standart / custom file format (Crono Cross / Legend of mana etc...) with indexing.

For bigger expantion of a file, I belive u habe to change the LBA of that file to be the last one in the CD (if i have still place on the cd :p).