You should use the placement.info as sebastionros implies.
Enable the shapetracing feature, and view a blogpost with an image on the front end of the site. Then use the shape tracer to see which part or field is holding the image.
Then you can just add a section in your theme's placement.info:
<Place Fields_YourImageField="-" /> <!-- If it is a field -->
<Place Parts_YourPartThatHasTheImage="-" /> <!-- If it is a part -->