A JSON-LD for SEO customer was asking me a few questions about his Shopify store's performance the other day.
The product pics I use in my shop range approx.. from 100kb to 1.2mb. Is it worthwhile to reduce the large ones to a smaller size? And what is a kind of "best size" when it comes to this?
The best size will depend on the max size of the images your theme uses, as in the width and height.
There isn't a best size in regards to the image file size though the smaller the better as long as the image still looks the same (e.g. isn't grainy or pixelated).
Shopify recommends keeping the file size below 70 kb but that'll vary depending on how many images you use on a page and how many colors are in the images (yes, images with more colors take more space).
What I recommend is contacting your theme vendor to see what the max sizes of images you should be using in height and width. They'll have designed their theme to work with specific image sizes. Some will even have used Shopify's built-in image resizer features to automatically use the best size.
By using their recommended sizes, you'll know exactly how big your images should be which will save on the file size which is the end goal.
If you're on an older theme, especially if it's a non-responsive one, you might want to consider replacing it soon. Responsive themes have pretty much stabilized these days so they are a major improvement over older non-mobile or dedicated mobile themes. Just schedule it at a slow period to minimize your revenue impact.
Most Shopify themes don't quality for Google's Rich Results so they lose out on highly visible SEO enhancements.
JSON-LD for SEO provides high-quality data that will let your store qualify for Rich Results. Often times faster than the industry averages.