You can "click" the checkbox. KM calls it "Press Button".
Here's an example.
I don't have your app, so I'm using Outlook 2011 Print dialog as an example.
Notice the checkbox "Pictures in message body" is UNCHECKED.
So, I use these actions to check it IF it is unchecked:
I just tested this and it works fine.
BTW, I really like, and sometimes use, the "Found Image" actions, but only as a last resort.
(sorry @iampariah, but I've got to call 'em as I see 'em. )
Found images are the least reliable means of finding/clicking on something on your screen. They sometimes don't work on other Macs and/or other monitors. If the App happens to change it even slightly, then it breaks.
Let us know if you have any further questions.