Is there a best time to post artwork so that it trends higher?
No there is not.
We often get asked the question about when is the best time to publish artwork. Artists think that there is a "magic bullet" - if you post at a specific hour of a specific day, your work will magically float to the top of the Trending wall.
It doesn't work like that. We often see great work trending no matter what.
It's really hard to predict when the right people are online and looking at the site. At any given time there are thousands of people on the site browsing. It's also affected by what's happening around the world - e.g. during the US presidential elections, many people were looking at that instead of ArtStation, etc. It's impossible to say that at a specific date/time, it will perform the best.
Also, if such information did exist and we told people to "post your artwork at this hour on these days", everyone would post at that time and it would negate the benefit.
So... no there is no best time to post your artwork. Just post it whenever you like. :)