In EW2009 we had stored many backgrounds which we used as logos.

According to one of your staff on the technical support line, there is no way to use "themes" as logos in EW6.

We have very few images stored so are going to be in difficulties selecting logos.

Is there a way to import our themes across to the Image bank, if they are only on the computer as backgrounds and not stored as images or ppt slides elsewhere?

Right click the theme you wish to obtain the image from.
Click on export theme file.
Save the file where you can find it.
Browse to the file, right click on it and rename it.
Leave the original file name and change the file extension to .zip instead of .ewtx
Open the zip file and open the media folder.
You can copy your image out of there and add it back into the media folder.