When you select a filter in most themes, it will show the default product image. This is fine in most cases, unless you have images that match your variant filters.
For example, if you have a filter for your Red variants, and red images on those variants, then you would probably want to show the red images, instead of the default blue image.
This does require some theme modification. We do have a theme installer for adding this feature in, however due to differences in themes, it may require manual installation.
The helper is called Variant image selected, in the Theme Installer:
You may need to follow the manual steps, depending on your theme, and you may also need to adjust the settings in the code snippet, as by default it only looks for color, size and material options.
If you have any issues, please contact support and we'd be happy to install it for you.