Hi Maria, the "VM - Custom Field, stockable variants" won't give you the option to change the product picture. Because that plugin is not capable to do it. (it is a Virtuemart thing, not us).
What you need is "Generic Child Variant". You will get a look of this product:
www.flexiblewebdesign.com/virtuemart-2-template-puremart/index.php?option=com_virtuemart&view=productdetails&virtuemart_product_id=50&virtuemart_category_id=6
At first, there is no "add to cart" button, because you need to select a product variant. One you select a product variant, the product picture gets changed, and (depends on the stock info) the add to cart button appears (or the "notify me" button appears). This is a better and more professional way to build a product with variant.
Since "child products" get involved, you will be able to keep the stock, and additionally you will be able to add different product picture to each product variant. So all in all, using "generic child variant" is the best.
Use "Generic Child Variant" always and forget about the "VM - Custom Field, stockable variants".
I haven't created a tutorial for "Generic Child Variant" to get the same look as we have in the DEMO but i will do it soon. You can play around and try to get the same look though. It is not that hard, furthermore, you can take a look at the
forum.virtuemart.net or search it through google. I am sure somebody has explained with screenshots how to create "Generic Child Variant" type custom fields. (The procedure of creating Generic Child Variant custom field would be the same with or without PureMart)
There is one thing you need to remember, while creating the custom field, you should:
ENABLE: Display parent as option
DISABLE: Parent orderable
So, parent product won't be purchasable, because it is a parent and it exist to be appeared in the "category browse page". If you have experience with more professional eCommerce software, (like: Amazon Seller Center) they follow the same procedure, the parent products are always "not order-able". Try to open any clothing product from amazon.com, you won't be able to add the product to cart without choosing the "size" or "color" variant. This is the only way to make it user friendly.
I found this website below just now. check it out, it provides screenshots about how to create "Generic Child Variant"
cocoate.com/vm2/product-attributes-custom-fields
And look at the section of "Creating the t-shirt products". Very usefull and easy to understand.
Regards.
Support & Development