If You are want to Add Thumbnail In Your Blog Post Like Youtube.So Follow These Steps:-
Step 1:- Open Your Blog Account And Click New Post.
Step 2:- Change Your Compose View To HTML View.
Step 3:- Create Your Thumbnail From canva.com or pixlr.com.
Step 4:- Convert Your Image Into Url Through atatus.com.
Final Step:- After Convert Image open your post and use
<img src="_Your_Image_Url" style="display:none">