Yes, the default Theme image is the one you describe when both quick look previews and Homepage Preview are unchecked. This will get updated as you open each project file (I believe).
My recommendation was meant to clarify that you should choose one or the other. Please note that I believe @nikf said that that image only gets updated weekly (I believe) when having the Homepage Preview option checked.
OK, so perhaps your saving issue is related to having multiple projects open at the same time and 1 or more timing out, this information is important to @nikf as it will assist him in squashing this bug once and for all. Please provide the details above to firstname.lastname@example.org.
Others are experiencing saving issues when in Preview mode but have not deduced it=f they have multiple projects open at the same time yet though… It could be one, both or either that is at fault or partially at fault for the save issue.
Others have reported bringing their mac back from sleep has caused save issues with left open projects.
Personally, I only experienced this particular bug once in 6.3.5 and has not occurred to me again and I do have several projects open concurrently, close everything prior to Mac going to sleep (Screensaver seems to be ok).
Anyway, I am not the programmer so have tried to help as much as able, please contact support and hopefully they can squash this bug once and for all.