Hi,
could you please remove the () brackets from the image filenames
had to use DietPi_blahblah-(Jessie).7z to use your filename in any command.
No, we wont be doing that 
had to use DietPi_blahblah-(Jessie).7z to use your filename in any command.
You can also put the file in quotations, removing the need for escaping backslash, eg:
wget 'http://dietpi.com/downloads/images/DietPi_OrangePiPlus-armv7-(Jessie).7z' -O image.7z