The present study aims to explore the factors affecting Korean EFL high school students’ choice of picture options with key expressions in a writing test prompt, as well as the effects of option position and contents on their performance. It further aims to examine whether these effects vary with English writing proficiency. The performance of lower- and higher-level students in two prompts was analyzed along with their reasons for their choices. The prompt had three picture options with two key expressions in each option; the students chose one picture option and wrote a 20-word message declining a request using a reason specified in the option. Significant effects were not found for position within the prompts, but for the content of the options. The participants tended to choose a certain picture option over others mainly because of topical knowledge, difficulty level, or picture preference. The findings suggest a significant effect of picture contents, which yields implications for designing prompts with picture options for score validity.