it's not an iPhone-specific problem.
You will need to edit this plugin's file: "./scripts/front-css.css".
If you don't have direct (ftp) acces to it, in your dashboard go to menu " >> Plugin >> Editor". Once in this page, at the top-right part of it, choose and select (click) the plugin to edit (Cnhk Slideshow).
Then now, on the right side of the page there should be a list of files. The concerned file is in the middle of the stack - the 11-th from the end - (cnhk-slideshow/scripts/front-css.css).
The first rule (the 1st block) in this file looks like:
You'll need to add "!important" to the last property. So the first rule should look like the following:
height: auto !important;
Don't forget to save changes by clicking on the "Update File" button.