Hi Chini,
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In most of our quickstarts is the same rule: https://www.joomshaper.com/documentation/joomla-templates/kreativa/no-photos-from-demo
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If your template (quickstart with demo content) is based on SP Page Builder, so images should be changed inside Pages > addons (for example Image addon).
I am afraid that Joomla basic skills are needed to customize any website / quickstart. If you are "green/beginner" user - it will take you some time (days, weeks) to discover everything. But try to use template documenation guide, and find Joomla teacher in your city.
Referring to the proverb that "Rome was not built in a day". Learning a web skill takes time and patience. This applies to any tool, whether Joomla SP Page Builder, WP Page Builder, Elementor, or Editor like Photoshop.