Shopify Dawn Theme: Demystifying 'Image Larger Than Needed' PageSpeed Warnings

Hey there, fellow store owners! PageSpeed scores are crucial, especially on mobile, and images often cause Lighthouse warnings.

Recently, an insightful Shopify community discussion emerged. Store owner Puneet (aka WetandDry) investigated a "Serve properly sized images" warning on the popular Dawn theme's product thumbnails. He found a fix, then bravely corrected himself after further investigation and community input.

Decoding the "Image Larger Than Needed" Warning

Puneet tackled the "Serve properly sized images" warning from PageSpeed Insights. This means your browser downloads an image significantly larger than needed, wasting bandwidth and slowing things down.

He initially suspected 823px in snippets/product-thumbnail.liquid's widths list, observing the browser picking this rendition on certain mobile viewports when a smaller one would suffice.

'sizes' vs. 'widths' and Pixel Density

Experts like ecom-4all and Taha_Dev offered critical clarifications. The widths list tells Shopify which image versions to generate; the sizes attribute tells the browser which to pick based on screen space and pixel density.

Taha_Dev highlighted that Lighthouse's mobile test needs roughly 650 pixels (e.g., 412px-wide phone at 1.75x density) and typically picks the 713px rendition, not 823px. Removing 823px could make devices needing images between 714-823px download the larger 990px version, potentially increasing file sizes.

How to Diagnose It Yourself

Before modifying theme code, Taha_Dev's diagnostic method is highly recommended:

1. Open product page with browser Developer Tools (F12).
2. Switch to "Device Mode."
3. Hover image in "Elements" panel.
4. Note Rendered size, Intrinsic size, and Current source.

If Intrinsic size is significantly larger than Rendered size multiplied by pixel ratio, you've pinpointed the problem. If it's a small jump, performance gain might be minimal.

Puneet's Corrected Fix

After further investigation and helpful community input, Puneet posted a follow-up, correcting his initial advice. He reaffirmed the issue lies in snippets/product-thumbnail.liquid where both image_tag calls include 823 in the widths list.

Here's the fix he proposed:

File: snippets/product-thumbnail.liquid

Find:

widths: '246, 493, 600, 713, 823, 990, 1100, 1206, 1346, 1426, 1646, 1946'

Replace with:

widths: '246, 493, 600, 713, 990, 1100, 1206, 1346, 1426, 1646, 1946'

He observed that removing 823px caused the browser to step from 713px directly to 990px, resolving the PageSpeed warning without visible quality loss. He also noted image-magnify-lightbox and image-magnify-hover classes are generated directly in this snippet.

Important Considerations

While Puneet's fix might resolve a specific PageSpeed warning, consider these community insights:

• The "sizes" attribute: As ecom-4all suggested, the ultimate lever for image selection is the sizes attribute. If your sizes expression consistently evaluates to a larger width than necessary, that's the ideal (though more complex) place for precise optimization.
• Theme Updates: Direct code changes in your theme files will be overwritten during future Dawn theme updates. Always record your changes.
• Test Thoroughly: PageSpeed results fluctuate. Rerun tests multiple times. Check your site on various real devices (or using DevTools) to ensure no unintended quality loss or increased file sizes.

This discussion highlights web performance complexities. While removing 823px might offer a quick fix, understanding why the warning appears (and the role of sizes and pixel density) is key to truly optimizing your store. Always diagnose with DevTools first, understand implications, and be mindful of future theme updates!