REPEAT/L=1:12 (FILL SST; GO fland; FRAME/file=sst0_`l`.gif)
should work! I copied and pasted it to my Ferret prompt and it did produce the 12 image files sst0_1.gif, sst0_2.gif, . . . , with right images. The puzzle is why it failed on Saurabh. Or am I missing something?