The growing popularity of automatically graded online tests, either as an evaluation or self-assessment tool in online or blended education, demands a review of how these questions are designed and delivered to their intended audience. This paper analyzes the results of over 20,000 preuniversity mock online quizzes designed to train the students for the Spanish university admission test (known as “Pruebas de Acceso a la Universidad” or “Selectividad”) in the technical drawing subject, corresponding to the June and September intakes of 2009 and 2015. The influence of two key aspects on the questions success and skip rates is assessed: (a) the presence or absence of illustration support and (b) the length ofthe question as a proxy ofreading comprehension difficulty. The results support that the presence of an accompanying illustration in the questions result in fewer skipped questions and mode successful answers, while the length of the question has the opposite effect. The performance difference in the 6-year span is also discussed, showing a slight decline over time in the pass rates while the skip rates remain stable. When comparing both two intakes, corresponding to different academic profiles of students that passed the June exam and those who did not, the success ratio is unsurprisingly lower for the students in the second intake. These findings should help improving the design of online quizzes, including more visual content and/or rephrasing the questions to be more concise, to fit the requirements of students educated in a more visual environment of multimedia technologies.