Your question is a little light on detail of what you want to happen precisely.
In general anything in WordPress content is usually treated as such. It’s not very friendly to being mixed with code, although many solutions try (often with poor results).
The basic approach would be to use the_content filter to process post content on display and insert modifications as necessary.
Other options I can think of would be inserting changes when post is save or customizing markup of image being inserted into content (image_send_to_editor filter).