Web video browsing is rapidly becoming a very popular activity in the Web scenario, causing the production of a concise video content representation a real need. Currently, static video summary techniques can be used to this aim. Unfortunately, they require long processing time and hence all the summaries are produced in advance without any users customization. With an increasing number of videos and with the large users heterogeneousness, this is a burden. In this paper we propose VISTO, a summarization technique that produces customized on-the-fly video storyboards. The mechanism uses a fast clustering algorithm that selects the most representative frames using their HSV color distribution and allows users to select the storyboard length and the processing time. An objective and subjective evaluation shows that the storyboards are produced with good quality and in a time that allows on-the-fly usage.