How to solve mobile usability error.Imag and text visible very small on mobile.

It's all a matter of the theme used. The included themes are not really mobile friendly except of the jquery mobiel theme. See the themes section for some third party themes that are.

Thanks Acrylian !
Pls recommend theme & also review https://paintingevent.com for your reference.
Thank You !

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Recommended mobile usable themes would be zpBootstrap, zpBase, libratus or zenji.

zpBootstrap works well, adapting across multiple devices and OS. Highly recommend.