How can I put a picture on my desktop - not as background

You can center a picture in the background by going to Settings > Personalization > Background and then in the Background dropdown menu select Picture, browse for your image, and entered Choose a Fit Dropdown menu select Center.

LesFerch said:

Overlap Wallpaper is a free portable app designed to do exactly that.

I've also written a small portable app that can display one or more static pictures, or one or more slideshows, over the desktop with no title bars (i.e. each picture or slideshow is a borderless image at a size of your choosing).

LesFerch said:

The portable app, I mentioned in my previous post, is called DesktopPic and provides that feature in two ways (picture displayed by app or picture stamped to wallpaper).

Thanks for that! What's the difference between the two, they seem the same? The first one does slideshow?

DedktopPic has two options: a slideshow or stamp an image to the background. The stamp option is zero overhead because it modifies a copy of the background image and then exits. The stamp option also supports images with a transparent background (see the sample showing the cat image overlaid on the beach photo). IIRC the other app overlays a single picture with no transparency support and the app has to remain running.