What type of cap is it? If it's a small disc cap, no. If it's a small metal cylinder, 99% yes, and it should be marked with a stripe on one side indicating the negative side. Most tantalum caps are polarized and you have to look pretty closely to see the "+" stamp on the very small ones. Tantalum caps are pricey and aren't used in kits very often. They look like a small tapered blob of hard plastic. If you're not sure what you have, can you toss up a pic?
I can't really see anything in that pic, it's pretty fuzzy. Do you have a pic of just the capacitor? On the board I can make out the triac and the opto-isolator, but not much else. Is the cap already installed?