This mixed-method case study was conducted to probe how a set of pictures had an influence on a group of EFL university students' retention of English words. Seven Thai university participants, enrolling in the course of English for Service Industry, were voluntarily engaged in the study. They took a pretest of 45 words they learned in class through the use of pictorial input for one semester, and right after the posttest, they recalled how they could remember the words in an individually stimulated recall protocol session. The result of T-test from Wilcoxon sign-ranked test showed that the pretest and posttest scores were significantly different at the 0.05 level. Interestingly, the qualitative accounts from the stimulated recall revealed that apart from the pictorial input the participants learned in class, they also employed other strategies to help them memorize the vocabulary. The findings from the study; therefore, shed lights on cognitive-metacognitive processing and strategies an individual EFL learner adopted, and most importantly, on how teachers can encourage their learners to orchestrate them and make the best use of pictures in order to learn ESP vocabulary effectively.