Compute cycles are a measurement designed by Liquid Web to measure the utilization of the Cloud Sites services. This primarily consists of CPU processing time but also measures disk i/o as well. If you are concerned about a large number of accesses to media files, there is a media accelerator available to reduce the amount of compute cycles consumed.
There is not currently a way to cap cpu cycles or disk space, so if you are concerned about going over the limit you will need to monitor your cpu cycles closely. Your compute cycle statistics are available to you in near real time from your Cloud Sites control panel. In addition we are happy to assist you in locating any problems causing you to use excessive cycles, just submit a support ticket.