Jo,
First, welcome to MODx! You’ve made the right decision
I assume you are on MODx evolution, right?
Now to your question, I’m not quite sure I get you but you seem to be asking how different documents can be assigned a different TV value, specifically an image TV?
1. Have you created the image TV and assigned it to a particular template?
2. Are you documents using that template?
If the answer is yes to both 1 & 2 above, all you have to do is click on the button (insert) beside the image TV while editing the documents and select and image. MODx will deal with the paths.
For each document, the image you selected will appear where you chose for it to appear, i.e. where you placed the image TV placeholder (e.g. <img src="[*menutv*]"/>). I hope this makes sense. Feel free to ask more questions.
Hope this helps
cheers/k
EDIT: corrected img tag code + corrected TV syntax
NOTE: If used in WF the TV is called thus: [+TVName+]. If outside WF, it is called [*TVName*]