@Joomla_Direct Yes, Captions is compatible with Joomla 1.5. A new version will also be released this week.
IFrame removed on saving or editor toggle
IFrame tags are removed when clicking the Save button or toggling the editor
Check that the Editor Profile Parameters allow IFrames
In the JCE Control Panel, click on the Editor Profiles button.
Click on the Profile name that the user, usergroup or component is assigned to, eg: 'Default'
Click on the Plugin Parameters tab, then on the Media Support tab.
Check that the Allow IFrames is set to Yes.
Configure Joomla! Article filters
By default Joomla! 'cleans' content on saving, removing script tags, event attributes (such as onclick, onmouseover etc.) and style tags.
A simple article parameter change is all that is required to fix this.