I'm a newcomer to the realm of queueing theory, so please bear with me :)

I'd like to model web server traffic with a modified M/M/1 queue. In the simple case we have two parameters - $\lambda$ for the arrival rate and $\mu$ for the departure (or service) rate.

If I understand correclty, the general way to get the performance evaluation equations (average number of requests in the queue, for example) is to draw a flow diagram, and solve the equlibrium equation system, namely for the M/M/1 model:

$0 = -\lambda p_{0}$ + $\mu p_{1}$

$0 = \lambda p_{n-1} - (\lambda + \mu) p_{n} +\mu p_{n+1}$, n = 1, 2, ...

I don't know how I could extend the model the fit the real-world scenario the most. Each normal request induces a number of image requests, for example, let it be $u$ on average, and let it's service rate be $\sigma$. How can I factor these into the equations?