Understanding the mechanisms behind high-Tc type-II superconductors (SCs) is still an open task in condensed-matter physics. One way to gain further insight into the microscopic mechanisms leading to superconductivity is to study the magnetic properties of the SCs in detail, for example, by studying the properties of vortices and their dynamics. In this work, we describe a method of wide-field imaging magnetometry using nitrogen-vacancy (NV) centers in diamond to image vortices in an yttrium-barium-copper-oxide (YBCO) thin film. We demonstrate quantitative determination of the magnetic-field strength of the vortex stray field, the observation of vortex patterns for different cooling fields, and direct observation of vortex pinning in our disordered YBCO film. This method opens prospects for imaging of the magnetic-stray fields of vortices at frequencies from dc to several megahertz within a wide range of temperatures, which allows for the study of both high-TC and low-TC SCs. The wide temperature range allowed by NV center magnetometry also makes our approach applicable for the study of phenomena like island superconductivity at elevated temperatures (e.g., in metal nanoclusters).