I haven't used the default image/caption approach, which might be the "correct" answer to your question … but pending a better answer, I'd deal with your requirement like this:

1. set up a style for images and a style for captions
2. inserting each image in an empty paragraph with the image style
3. add a caption after each image I want to include in the ToC, using the caption style
4. add the caption style to the ToC definition

In general, styles are the key to customising tables of contents.