It's possible to create the ToC directly within InDesign, but the process is so fiddly it's hard to explain in words (this is why those video tutorial websites make so much money).
But since you already have the ePubs made, it's probably easier to fix this in Sigil. Here I'm going to assume that you've made the ePub simply by placing jpegs into InDesign and exporting it. This will mean that you have a file with lots of
Code:
<div class="image">
<p class="para-style-override-1">
<img width="***" height="***" src="images/***" alt="***"/>
</p>
</div>
tags. If your file structure is different then this won't work and you'll need to show us what it looks like.
Open the ePub in
Sigil, and select View->Code View from the menu.
Now select Edit->Replace to get the Find/Replace dialog and type in the following exactly:
Find: <div class="image">(.+)</div>
Replace: <h1 title="Chapter X">\1</h1>
Set the checkboxes for Regular Expression and Minimal Matching, then click Replace All. Now go through your file and change all the new parts saying "Chapter X" to some text that describes the image, or just "Chapter 1", etc, Only edit the text inside the quotation marks, don't delete them.
Finally, make sure the Table of Contents pane is showing by clicking View->Table of Contents if needed, then click the 'Generate TOC from Headings' button at the bottom of that pane. It'll show a dialog that shows the new ToC, and click OK.
Before saving, go to the Book Browser pane on the left and open the Images entry. Right-click on the book's cover image and make sure that 'Add Semantics->Cover Image' is checked.
This will produce a ToC that assigns an entry to each image, which should be good enough for Lulu.