There's lots of different software out there available for use to create image or sound files. So long as they export the file formats that we support (png for images and m4a or ogg for sounds), just about any software will work.
There's also a lot of sites where you can download already created assets in the needed formats.
Here's a list of some options you can check out (some paid and some free):
Creating Your Own Images
Photoshop is an industry standard and has a ton of functionality. It works on both Mac and Windows computers, but is not free.
GIMP has functionality similar to Photoshop, and also works on both Mac and Windows computers. The upside is that it's free.
A great free, online, tool to create pixel art.
Paint.net is a little simpler than Photoshop or GIMP, but that will make it less daunting for people who haven't made their own graphics before. It's also free, but only supports Windows.
Creating Your Own Sounds
Audacity is a nice free option for sound creation, and works on Mac and Windows computers.
Leshy SFMaker (https://www.leshylabs.com/apps/sfMaker/)
Leshy SFMaker is a sound generator, which is much more approachable than something like Audacity, as you simply adjust values to get a sound you can use.
If you'd prefer to download already created assets for your projects, check out these resources.
Soundimage.org has a ton of royalty free sounds and images available for download. It's a great resource if you're looking for sound effects, music, or backgrounds.
This is a place where you can download all of the assets used in the curriculum for use in other projects. There's also a lot of assets available here that we don't use in the curriculum (check out the "ocean" and "large variety" asset packs).
https://gshelper.com/ - Non-free option
This is a marketplace you can purchase templates and assets from.