Apparently, we can find solutions to most of our errors in this document.
I followed the instructions: Use experiment settings > online > +/- keys to add the list of resources your experiment will need (2020.2.6 onwards).
For those who don’t know yet, the experiment settings on the top panel of the builder. Once you added your images, synchronize your experiment again. Then it should work.
However, the pictures never get fully synchronized for my study. So I had to manually add the resources that I need to the JavaScript codes. The resources should be added to this section of your JavaScript.
**psychoJS.start({
expName: expName,
expInfo: expInfo,
resources: [
]
});