Why does image quality decrease after uploading to my online store?

Store Design
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Topic summary

Main issue: After uploading 1920×1080 JPG images to the store, visible quality degrades despite the user believing ratio, size, and format are correct.

Mobile-specific: A second image is set for mobile, but maintaining aspect ratio fails when switching the theme’s image size options (small, medium, large). The result looks correct only on some devices (e.g., desktop or fullscreen), not consistently across mobile.

Setting “adapt to first image” breaks the second (mobile) image, worsening display consistency.

The user requests troubleshooting help and provides the store URL (https://imka.cz) for review.

Status: No fixes or guidance from others yet; the issue is unresolved.

Summarized with AI on January 6. AI used: gpt-5.
1

Hi i want to ask for help. My imiges keep loosing quality after i upload to my store ratio and size shud be fine and format too (1920x1080,jpg) but wheanever i uploud it it looks bad ( like quality) drops down, and i don´t know why.

and for cesond i set to be my second img to be mobile img but i am still strougling with ratio when i use deferend sizes in down theame ( small, medium and large ) it keeps beeing good on only some devices only on desktop or fullscreen and when i set ,adapt to first imige, it totaly break up my second (mobile) img.

Please help i keep struggling with this for long time.

https://imka.cz