You won't just have 15, I have like 32 or 33 with the paid account. LJ faqs say you get 30 with a paid account and then one for every three months paid time you (or someone else has bought) and then one bonus userpic for every continuous 12 months of paid time applied to your account.
Anyhow, you won't have just fifteen, you'll have 30 something of them.