Just checked, it confirmed:
Have your images named in some format. example: 1-9 as 1-9, 1-99 as 01-99, 1-999 as 001-999, etc.
So lets say you have have 2000 frames, so you need to go from 0001 to 2000, so Map_0001.jpg to Map_2000.jpg
Then, in terragen load one of them, doesn't really matter. Now change the filename of the map to
Path\to\directory\of\images\Map_%04d.jpg.
It should now auto load 1-2000 depending on the frame.
But lets say you only have 2000 frames, but named your images Map_00001 to Map_02000.. it wont load those as is!
Simple fix

, change the %04d to %05d.
The %(number)d allows you to choose how many leading zeros to use on numbers under a certain amount. So %01d would be simply 1,2,3, etc, %02d 01,02,03....