I had struggled with this for quite a while until I figured out why the Roku TV was not showing the thumbnail pictures. I had posted on this previously - somewhere around here?
The issue is that the older Roku devices - standalone or TV - did not have enough memory within them to support the downloaded code within them getting larger. The code getting larger comes from two sources: as suppliers update their code from time-to-time they inevitably make the size of the code grow - and - as time goes on you add more access apps to your Roku to get more channels.
Both of these use more memory. When memory gets insufficient, the actual operation code of the device is protected so that it at least continues to work. Sacrificed is the nice pictures in the thumbnails.
The Solution. You cannot do anything about the bloat occurring in the apps, only the channel software providers can do that. But you can delete all those loaded apps that you don't really use or want. When I started the process, I had 23 apps loaded. I deleted entirely 12 of those, leaving the "essential" 11. Reset.
The problem went away immediately and has never returned.
Also, remember that if you change your mind and want an app back, you can re-add it at any time.