You can make a new post through the main navigation or using the navigation at the upper-right corner, like this. You can use the QuickPress feature, but you won’t be able to set thumbnails for the posts.

If you want to put the post in a category, just choose one (or more if you like) from the Categories list. You can add a new category if you think the post doesn’t fit in either of those listed.

For the thumbnails, I suggest using a square-like sized picture, than a rectangular one. You can set the thumbnails using the Set Featured Image feature.

After uploading, you will see a screen like this. Just choose the Thumbnail size and click Use as Featured Image. Don’t click the Insert into Post button since we only want the picture as a thumbnail. Then click Save All Changes.

If done correctly, the picture will appear in the right side of the dashboard.

To publish, just click the Publish button and the post will immediately be placed in the homepage.