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It's either the images are blocked (by their firewall or antivirus or country filtering like China) or it's a glitch in the form. Ask them to check their browser console for logs.
1. While viewing the form, press F12 on the keyboard to open the developers tool.
2. In the developers tool, find and click the "Console" tab. The logs are in the console area, red texts are errors.
I checked your last form ID and can only find images that are related to the form backgrounds. Are these the images that you are referring to?