I want to download a Sprite in Runtime. But what is the url of the Sprite?
What do you mean? You want players to be able to download sprites that are in your game while the game is running? Like they click on a sprite and download it?
If you want sprites to be dynamically downloaded by event within the Construct action sheet sequence, then you have to have a blank placeholder sprite and reference the URL of where it is stored.
If you are asking about where native sprites are stored, it depends on what you are exporting in. In HTML5 exports, they are stored in .../exportDirectory/images (assuming you use the default option for "Subfolder for images") otherwise it will be in .../exportDirectory/customSubFolderForImagesDirectory where the ... is whatever you have in the "Export files to:" field.
If you are asking about where the Construct client keeps the sprite image files in preview mode, I believe it just references the image files directly on your hard drive when you initially created the sprite.
Use the "Load image from URL" action of the sprite object.
The URL is the URL of the file itself and is the URL to any file you've hosted online for this purpose.
If you don't know what an URL is I'd suggest you to first have a web search engine look at the meaning of this acronym in your language, this might help you.
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For my project the sprite is changing at runtime and I want to download (browser) the fine sprite image at the end of layout. What I do?????