Hi Christina,
One solution is to use an Opportunity Analysis report with the Lead Source attribute. Please see example screenshot below.
To avoid double counting opportunities, the opportunity "credit" is split equally. That is why the report shows fractions. For example, Blog is only getting 1/3 of the credit for the first opportunity because out of the three leads/contacts associated to this opportunity only one came from Blog. The other two came from other Lead Sources.
One last tip when you create this report: Don't forget to put a time filter first, e.g. Opportunities Created Month = November 2013. This will limit the data and give you better performance while you customize your report. Once the report is configured the way you want it, you can increase the time frame and let it run.
TJ