Multimedia Sensor Networks (MSNs) have gained much attention in recent years from the emerging trends of Internet of Things (IoT). They can be found in different scenarios in our everyday life (e.g., smart homes, smart buildings). Sensors in MSNs can have different capacities, produce multiple kinds of outputs, and have different output encoding formats. Thus, detecting complex events, which requires the aggregation of several sensor readings, can be difficult due to the lack of a generic model that can describe: (i) sensor networks infrastructure, (ii) individual sensor specificities, as well as (iii) multimedia data, while allowing the alignment with the application domain knowledge. In this study, we propose Multimedia Semantic Sensor Network Ontology (MSSN-Onto) to ensure MSNs modeling and provide both syntactic and semantic data interoperability for defining and detecting events in various domains. To show the readiness of MSSN-Onto, we used it as the core ontology of a dedicated framework (briefly defined here). We also adopted MSSN-Onto in HIT2GAP European Project. A prototype has been implemented to conduct a set of tests. Experimental results show that MSSN-Onto can be used to: (i) effectively model MSNs and multimedia data; (ii) define complex events; and (iii) allow to build an efficient event querying engine for MSNs.