I'm trying to build a small C# Windows Form, graphics program and I need to save/load sprite sheets. I've searched but can't find a standard file format for sprite sheets, I have however found the .sprite extension.
So my questions are:
I have personally never heard of a .sprite file extension; Googling it seems it may be used for the Scratch language (https://en.scratch-wiki.info/wiki/Scratch_File_Format#Sprite_Files).
Typically though, spritesheets are just image files with multiple images on them. PNG files seem to be the most common, though there is no real reason you couldn't use basically any image file type (though keep in mind some may not have alpha channels, etc.). Most software that packs spritesheets will output in a PNG, and will either split it up based on a grid system or provide a catalog of information (often in JSON format) for splitting the sheet into the subimages you need.