Atef, I am wondering what you mean by "image." Do you literally mean photos or some sort of visual imagery of international organizations, or do you mean their representation more broadly? Also, what kind of international organizations are you talking about?
Ivo Carneiro de Sousa is right to point out that the theory you use will depend on your research questions/objectives. In general, though, framing can be a useful approach. You may also consider the news flow theory (Wu), which focuses on the structural factors that influence international news coverage. Theories like indexing (Bennett), cascading activation (Entman), and propaganda model (Herman and Chomsky) have also been used to study international news. I think they may be applied to study the coverage of international organizations as well, although each of them has a different focus and addresses different types of research questions.