The GMetri file manager handles sprite as a .zip file. To create your own sprite file, follow the following steps.
All the sprite frames need to be named in sequence. You could number them as 01.jpg, 02.jpg and so on, or alternatively you could label them as a.jpg, b.jpg similarly.
Create an empty text file titled sprite.txt.
Compress the above mentioned files as a .zip file.
You have successfully created a sprite file. This file can be uploaded to the GMetri file manager and can now be used in any GMetri experience as you please.
Please ensure that an empty text file named sprite.txt is present in your zip file. This is so that the file manager can identify that the zip file is meant to be used as a sprite. If this step is missed, when trying to use the file as a sprite, the file manager will display the file as not supported.