I would stick with the picture flashcards because they are much simpler than the books. I would also try different interests the child may have such as Thomas the Train or football. You could also use candy as a motivator. You want to always give the student the language you want them to use. Students with autism learn at a much slower rate than typical students so you need to take that into consideration. Also they are generally not motivated to do school work so they may always seem unengaged but that doesn't mean that they are not learning. I worked in a autism school and we used the VB-MAPP as a tool and guide to teach sever autism students that had no language.

Some districts have a computer program called Imagine Learning. The program speak in home language and target language and adjusts the questions to grade level. Also, powermylearning.com will allow for games that align with the standards.

I like your already mentioned idea of picture books! I would say to help engagement, find what the student is interested in and then find picture books on that. By using pictures and activities based on what the student enjoys can help the engagement! Also, giving the student options and having them point to their favorite option, so they feel like they have a choice in what you are doing.