Whilst I have been using Painter 12 for years, I have rarely sought help because I don't like pestering people, but this is an issue I still haven't really been able to solve! It probably sounds a bit silly but bear with me!
Basically I have found that for a long time, when working on a RIFF file, it always seems to look really sharp and rich on my monitor but when I save to JPEG, even 100% quality, it seems to look really washed out with windows image viewer. To compensate for this, I've often tried adjusting the contrast, but this often creates a discrepancy between monitors. For instance I could view it on my monitor, my parents' computer and my phone and it would look too dark on one but fine on others.
To try and give you a clearer example, if you look at the boxart for the new DooM 2016 game, you can see that whatever format the image is in and whatever monitor you view it on, the image is consistently rich and sharp, whereas I am getting discrepancies across the board. Obviously it has been professionally produced by a trained digital artist which I am not! But I figure if they can do it, it must be possible.
I'm sorry this probably sounds a little convoluted, any advice would be helpful and if you need me to explain anything more clearly just ask!
What are the riff thumbnails? I saw that a thumbnail program was installed with Painter 2018, but was thinking it had to have been some type of separate program, but I am beginning to think that it is simply the thumbnail that is produced, which can be seen in the windows explorer where files are organized by storage....?
Are you using the Cloud for storage? If so, you're maybe seeing thumbnails stored on your computer? I'm only guessing at this - I don't use the cloud!