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 will require some theme modification. We have an automated installer for adding this feature in, however due to differences in themes, it may require manual installation.
The helper is called Variant image selected, and can be found in the Theme Installer, under the "Additional Code Helpers" section
The installer will also add some new theme settings to your theme editor, under "Variant Images" - these settings will allow you to select which variants the helper should show the images for. The default options are color, and material.
You may need to follow the manual steps, depending on your theme. If you have any issues, please contact support and we'll be happy to help.