Unfortunately, many schools don't yet have infrastructure that's strong enough to support a multitude of devices in the classroom. Generally, this means that when all the iPads/Chromebooks in your class get connected to the network, it essentially chokes and can't download the images properly.
Unfortunately, there is no short term solution to this issue aside from re-starting the app or reloading the webpage. The long-term solution is to ask your tech department for help, namely an improvement in the infrastructure of the school!
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