Be very careful with ephemeral. It will delete the file after it has been read into memory.
That is before the image is finished processing.
You probably want to ensure the image is fully processed and written out (with a vaild image) before deleteing the original image.
Ephemeral: is used more to inform a controling process that the reader has finished reading the image and can thus be left on its own. It is used for example with a "show:" delegate to launch "display" commands in the background. Once "display" has read then image, the main process can continue with what it is doing.
I have also used it to create slideshows where the next image is not determined in advance, or the next image is created while the old image being displayed. Using ephemeral on a symbolic link pointing to a image, is also useful as it will remove the symbolic link and not the image pointed to.
NOTE: There is currently no way to get "animate" or "display" to signal when it has finished an animation, or has actually put the image up for display.
However you can have "convert" read a seperate "ephemeral:" image, to notify a controling script that it has reached a specific point in its image processing.
I have added some of these notes to IM examples, File Handling, Ephemeral
http://www.imagemagick.org/Usage/files/#ephemeral