my thumnail only appears in the editor but doesnt appear when veiwing it

this is my script

when green flag clicked
forever
show
set [ gohst] effect to (100)
go to [front v] layer
end

an easy script you can use (assuming that you have a play button coded) is the following

when green flag clicked
forever
switch costume to [your thumbnail v]
wait until <mouse down?>
switch costume to [game bg v]
end

Whenever you save a project, the look of the screen when you save it becomes the thumbnail, unless you are using browser extensions.

make sure that they also go back to the same spot and same size when clicked

Using ghost effect will make whatever sprite Invisible. It's pretty much the same as using the hide block. The only difference being that when the stop sign is clicked, the object with the ghost effect applied will become visible, and objects with ghost effect on can be hidden while still detectable by other sprites.

To help me help you further if your problem still persists, explain to me what you're trying to achieve.

It is also possible to use animated thimbnail from worldlanguages https://worldlanguages.github.io/animatedThumbnailsBookmarklet/ there's many projects with tutorials by searching “animated thumbnail”, but if gif is used for the thumbnail instead of image and the thumbnail is animated such project cannot become featured project if it's still true…

here: a very easy script is this (make the thumbnail first, but make it a sprite. not a backround)

when green flag clicked
hide

easy, right?

This is how I do it.

In the thumbnail sprite:

when green flag clicked
show
set [ghost v] effect to (100)
forever
go to [front v] layer
end