using "frame" repeatedly with the same file name produces files with an additional extension. Just use the unix command 'ls' to explore this.
What image files are suitable for you depends on the software to compile the animation. There is a simple old tool 'whirlgif' to produce an animated gif. Its nice and easy to use, but vanishs out of sight of google. It requires gif input. Please note, the most favourable version of ferret, namely pyferret does not support gif anymore but produces png-files. If you need gifs just write a little shell script using the ImageMagic tool 'convert' to produce gif files. Something likefiles=`ls *png`
for file in $files
More complex tools like mencoder can be used to glue a sequence
But now to ferret. Just organise a loop over the time slices. It
is easy to generalise it for more than 12 steps.
You may guess the reason for the complex way to build the filename. Using ls *png produces the file list in the correct order, not only for 12 time slices but also for a larger number (<1000 or for more with zw=4). This allows to produce an ordered list of files to be glued to an animated file with a single unix command ls sst*.png > filelist.
Hope this helps to get you started.