Try:
str = "title_" + t{n}
saveas(gcf,str,'jpeg')
This will attempt to convert t{n} into a string and concatenate the string "title_" and that newly created string. I've defined it as a separate variable so you can see what it looks like. If this doesn't do what you want, show us the value of str that is displayed when you run this code and describe how it differs from how you want it to look.
