I guess I have a good Idea for a new application. But I'm myself unable to handle it.

What about creating a configurable Sharing Plug-in for the Image Application?

The purpose of this application is to open a choicable external Application with the path of the currently viewed image.
It should be simple to create it with good C++ knowledge. I guess you can find everything what is needed here: http://wiki.maemo.org/Documentation/...ug-in_Creation

I often had the wish to open a image in a external editor. At example to open a photo in MyPaint.

Then we only need to persuade the MyPaint developer to make this command in the shell working with his application: mypaint %1