Ok, so let me explain what's going on: Your images (at least the ones you are referring to) include a specific color profile, embedded into the image. When X3 creates thumbnails, it doesn't by default also copy the color profile into the thumbnail. That's why you see difference between the preview images and your original image, as the image fades from preview-to-original on zoom.
Not long ago, it was recommended to ALWAYS convert your images to sRGB when you are going to use them on a website. WHY? Because most browsers did not support color profiles, or only a few, which means browsers would display your images differently, or simply wrong. That is why there is an option in Adobe Photoshop "Convert to sRGB" when you save/export images, and that is why it has normally been recommended to convert to sRGB for all your images intended to use for the web.
https://www.naturefocused.com/articles/ ... ernet.html
Times have changed, and now most browsers will support color profiles. However, this is not handled automatically by X3, for two reasons: 1. It's still not a bad idea to convert to sRGB for the web for cross-browser consistency, 2. It's not efficient to "copy color profiles" into X3 thumbnails, although it can be done.
TWO options for you:
1. Go to
Settings > Advanced > Image Resizer > and ENABLE
Copy ICC color profile, This will copy your original image color profiles into the resized images, so they should match. PS! You might have to clear X3 image cache and browser cache to see it work on existing images, but it would work immediately for new images (after applying the setting).
2. Follow the "traditional" fix of converting images to sRGB preparing them for a website.