Hi,
1- I don't understand your first question.
2- The image size should be large enough, at least 2x or 3x larger than the thumbnail resolution. If your thumbnail (depends on your website layout) is 400x400 then the original product picture must be at least 800x800. The optimum is 900x900 or any larger.
NOTICE: the "thumbnail" i mentioned above, not the thumbnails your Virtuemart have created but the small picture that appears in the product detail page.