Try Canva. It's one of the best free graphic design tools for designing advertising, motion graphics, and decks. I do not work for them nor do I have a commercial interest in them. Just like them. You'll possibly have to go to a paid account for 4-8k billboard sizes.
Also I second the suggestion of Inkscape, but it's only really for vector graphics (angles, shapes, like Adobe Illustrator), not photos. If you want a Photoshop clone, check out PhotoPea (online tool). For something with more power than PhotoPea, but a higher learning curve, check out GIMP (Gnu Image Manipulation Program).
Inkscape and GIMP are free and open source.
Last, for doing full blown movies (using video, sound, motion graphics), I like ShotCut (open source) for its ease of use and DaVinci Resolve (free) which is basically pro video editing software that competes very well with Adobe Premiere.
Since this is my first post to this forum, I am not including links, but a quick search should find these things easily.