::Not a stupid question at all! If you are drawing your own art assets, it is best to organise them into sprite sheets. Define a size for each sprite (for e.g. 128×128 pixels per sprite), then ensure that all the animation frames that you draw conform to that size.
Once done, you can use something like Sprite Sheet Packer to combine them into a single sprite sheet: https://www.codeandweb.com/free-sprite-sheet-packer
If you are looking for alternative art assets, you can look around in craftpix.net or opengameart.org.
i hope this clarifies things!
There are quite a few things regarding art that is difficult to express over text. I will be doing a stream on YouTube around 4AM GMT tomorrow. If you are free, you can drop in with any further questiosn you may have.