[TYPO3-english] Add a field to existing backend extension

[Andre]

Hello,
 
I'm trying to modify existing extension, but cannot find any manuals how it
should be done. Extension itself add a tab for some of the content elements.
All I need to do is to add a field where it would be possible to select an
image. And then in the frontend I would need to use that image. Can you let
me know what steps I need to take to make it work or direct me towards a
manual or an example of how it's done.
 
Thanks,
Andre