The TV doesn't have a hard drive. It's possible that some images are cached, but it's most likely that any trace of them is gone when the power is removed. Roku devices have very limited storage and most things are only kept in RAM which would likely be erased when the device is restarted and certainly erased when the power is removed.
For account information, that kind of information is stored in non-volatile memory by each app. Removing an app and restarting the device erases any data the app has saved and a factory reset erases all data. There is a caveat to this since some people have reported that after a factory reset their account information seems to be persistent after linking the device to the original Roku account. That would be a different mechanism at work and the information is tied to their Roku account and isn't being retrieved from the device itself.
Bottom line is that if you're getting rid of a Roku device, factory reset it and unlink it from your Roku account at https://my.roku.com and you should be pretty confident that no useful information can be retrieved.
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