Well there are two ways to approach the slider –
1. You can go to “Site Features” on your dashboard, “Add New”, and there enter the title and the content you want to appear on your slider. Then “Add Media” – find the image you want to use, and be sure to set it as “Featured Image.” (i.e. don’t hit insert to post). You should see it appear under “featured image” on the right. You’ll need to do this for each sliding image.
2. You can use the “Post categories” option – which I think you are using, and select a category, and pick up the featured images of each post in that category. I just tried that (a little embarrassed I never tried it this way), but you’re right – it didn’t work for me.
I know we have a bunch of changes due out with the next release – in a couple days I think – and some relate to the slider functionality. I will let the developers look at this issue.
Thanks for testing!
These options are controlled in the CBOX theme settings, under Homepage>> Slider Options.