Hi I got this to work for me with external image url link..
I use the Example VirtueMart Product import,
--File option tabAuto detect delimiters = yes
Use headers/nodes as configuration= yes
--FieldsRemove all, nothing should be there
--Optionsdoesn't really matter what you have there
--Image----General Options
Process images = yes
Create image name = yes
image name = product SKU "if you use the product name and your product name has white space it will messup the image url,, like if your product name is "product test" the image name will be "product test.jpg" and browsers can't read white space(%20).. so i would advice to use the product sku
image format, and Change case = it depends on you anything would be fine
-----Full image options
Keep original file = no
Image name format = no
Save images on server = yes
Resize large image = yes
width and height = your choice
--Pathimages/stories/virtuemart/product/
here's the output
http://testserver.websitedesignparramatta.com.au/index.php?option=com_virtuemart&view=productdetails&virtuemart_product_id=38#prettyPhoto[attachment cleanup by admin]