Hi,
I am the author of the redux theme. I have included a setup guide in the theme pack which will guide you to get all the aspects of theme up and running smoothly.
I can see on the post above by johnbablex has quoted the exact steps to enable thumbnails on your posts.
But before uploading any images, just make sure you have un-checked the upload organizing in month year basis.
You can do it by going to Admin panel > Settings > Miscellaneous and UNCHECK the organize uploads into month- and year-based folders.
This is beacuse the path to image is predefined on the theme and month /year variable will break it.
If you have already uploaded the images, i suggest you upload them once again after this setting is made.
when you fill the custom field, make sure the key is homethumb and the value is name of the image without extension like .jpg, not the url of the image.
It should work by now.
thanks
Jinsona