Image resolution & size for zoom effect in Sparkle

I am making Sparkle image galleries for a website.
Photos in some galleries zoom nicely and some hardly zoom (enlarge) at all.
I know this has to do with the resolution and/or pixels in the image so what are optimal sizes for images in the gallery to zoom to 2x and still keep the file size small enough so the page loads efficiently.
What is a good size for the initial image to have it zoom 2x)
Can I change the image size to make the image zoom larger without affecting how the image appears in the gallery?
Sorry, I understand some of this but for a Sparkle gallery it is not entirely clear why this is an issue?

You were on the right track when you said this has to do with the images’ resolution.
What you need to do is spend some time in Photoshop and make all of your images the same size and same resolution… then insert them into Sparkle.
I look forward to seeing your website’s gallery!