Please help, my posts have no thumbnails!?

So in 3 of posts I made there isn't showing a thumbnail, which sucks, because the pictures I'm using are pretty decent (imho). With my last post (https://steemit.com/cars/@watchesdaily/when-i-suddenly-had-a-lexus-rc-f-for-a-few-days) I even used a bit of a lower quality.

I normally upload photos from my camera, to my phone, edit, and then email them in the highest quality to my email. This time I used the second highest quality. Still no thumbnail shown. Is it really about the quality of the pictures or am I doing something wrong?

In my first few posts it just used the first picture in the post.

Please help me, now my posts are like a blind date, you're getting lucky, but it's still scary to click on them haha

Kind regards,
Ron