Are these considered as 'commercial'? i.e. if the picture uses items (perhaps freebies) designated as 'for non-commercial use only' would using the image for this purpose be 'illegal'?

*These images will be used for marketing communications only.  This will not violate that stipulation.  They won't be sold or submitted elsewhere for such purpose

You retain the copyright holder for your image, you are only allowing Renderosity to use it.  In order to give Renderosity permission the image is required to be 100% your own using items that allow for commerical work.

Wasn't sure about the points above. Now it's clearer. Thank you.