Questions

On the Image Edit page (NETWORK>Images in your Netcells User Zone) choose background colour from the 'ibg' droplist (beginning of the third line - this shows '- default -' by default).
Press 'Send'.
If you feel you must have a particular colour for your images, just send us its Hex code (ie. #FFCB9B) and it will be added to the list.

The background colour feature can also be implemented on your external site, if it is hosted with Netcells.
The default background colour for your site can also be specified in PUBLISH>SiteStyle