You're right - you can't use cron for anything less than one minute intervals.
However, that's not to say you have to use cron at all - a simple shell script with a
sleep 15 inside a loop would do it.
One consideration, though, is that screenshots aren't the best way to capture server activity. A lot of things can happen in a 15 second interval.
Therefore I recommend evaluating what it is you're trying to achieve, then seeing if there's a better way of doing it.
For example, if it really is screencasting/recording you're after, there are a number of utilities that will do this such as
Snapz Pro or
ScreenFlow - both have the ability to record screen activity and you can control frame rate, etc.
If it's just cpu activity you're trying to capture (e.g. which process is using all your CPU resource) then just run
top which will show more detail than Activity Monitor, can be set to update on any interval and its output can be written to a file just like any shell command, e.g:
top -l 10 -s 15 > /path/to/log
This command will run
top and capture 10 samples of data at 15 second intervals. The output will be written to /path/to/log, and each sample includes a timestamp at the top.
