Can't assign additional image

[joelgraham]

I have selected the 'additional images' from system/config and in the array have my images assigned:

[image]     => 'image.png',
[thumbnail] => 'thumbnail.png',
[small_image]   => 'small_image.png',
[image_foo] => 'additional_image.png'

All the images upload correctly, the image, small_image and thumbnail all assign correctly, but the additional image (image_foo) doesn't.

[joelgraham]

I've tried troubleshooting this error, too. Again, I can't see any discernible reason as to why these additional images wouldn't assign. Has anyone got any experience in assigning additional images, and any specific format that one might have to conform to?

Many thanks.