I am little confused.
Do you want to change the product-images based on selected Custom Fields? If you want to use "AutoSwapPicture" feature along with ZoomEffect, then you MUST disable the .chosen() feature that comes Virtuemart by deault. So the select-boxes will become ordinary select-box (Technically speaking; select-boxes' change() events will be captured by ZoomEffect)
So, there are few alternatives you may follow:
1- Navigate to "VM Backend >> Configuration >> Template" and disable this option: Use jQuery chosen for dropdowns in FE
(Which will disable the .chosen() plugin from select boxes and AutoSwapPicture feature of ZoomEffect will work)
2- Navigate to "Flexible Zoom Effect" plugin and disable this parameter: Auto Swap Pictures
(Which will disable the AutoSwapPicture feature but will let you run everything smoothly)[/b]
If you are using "Multi Variant" custom field, then you don't need the AutoSwapPicture feature, so you should safely go with the bullet#2. Because "Multi Variant" custom field uses "Child Ppoducts", so you can upload different pictures for different child products (which basically change the product picture after choosing differetn variant)
AutoSwapPicture Feature is mainly developed for "Image-Type custom field". Because image-type custom field doesn't use "child-products".
Regards.