By default Hekate looks for /bootlogo.bmp, however some repacks overwrite this path, in which case you have to check your /bootloader/hekate_ipl.ini for specific location. For example, Kosmos overwrites the path to /bootloader/bootlogo.bmp.
Magical tool to convert any image (PNG, JPG, bad BMP) to Hekate's expected bootlogo.bmp format. Just feed your file, cross your fingers, and hit run.
Are the files stored somewhere?
No, since all of the conversion is performed client-side, there is no need for a server. The only exception to this is direct URLs, due to nature of browsers, they have to be proxied through a 3rd-party service (which doesn't store anything either, it's just a universal proxy).