images disappear from presentations

images disappear from presentations

Hello Zoho, 
again and again, I am encountering problems with disappearing images from my presentations in Zoho Show.
As an art history teacher, I need to make pretty large presentations with a lot of images. And every time I re-open a presentation after a while (a few weeks, a few months, a year), several images have disappeared and are lost. Of course this is enervating, but also very time-consuming to search for the images again on the internet, while I am preparing to give my course the same evening. 
What can be the reason of these disappearances? Are the presentations too large? As I am working with a Chromebook laptop, I cannot go downloading every image I need because that would be too much for the laptop.
I am encountering this problem already for a few years, but it is getting quite frustrating now, as I have more and more lessons to give. 
Thank you in advance for your return.
Kind regards, 
Frank