OK, After some research and since Domain_views module seems to be a little bit buggy (printing always default Domain ID no matter what), I think this is the base question solving my problem the other way around with 2 domains and 2 themes using the same nodes from same node-type, but they both need different image-styles because of their 2 different themes:
How do I alter the given image-style of a given node-type in its default full-node view-mode in the .tpl files of a theme, to have 2 different used image-styles in 2 different themes by the same node-type and the same view-mode?
I've tried to find example themes doing it and I heard of Entity view mode to add custom view modes, but I am still not sure if I am on the right tracks. Actually I only need another image style (size) for the same node-type and the same view mode but in 2 different themes since Domain Access gives me hard times to solve this with conditional view modes or Views displays.
From what I understood there are 2 ways:
a custom view mode for the same node type and a function in the theme telling the node-type to rather use this view mode
or only a image-style altering in the node--full.tpl.php but I can't find a good example explaining it.
The very basic question to research for me is: how to alter image-styles in the code of the theme layer? Is this correct?
Any help would be very appreciated! Thank you very much for any hint.