Ok, there are many different ways to access thousands of official Pokemon artworks and sprites, so I will list a few that come to mind immediately.
One of them is the archive from Bulbapedia. This link has the official Ken Sugimori artwork of every Pokemon, (the art that is the header of every Pokemon's entry on this site,) as well as most of the official artwork from every single Pokemon game. This includes trainers, promotional images, and so on.
On the entire rest of the spectrum, this link will take you to the Pokemon Home sprites that are used in the app. If you use the menu on the left of the screen, you can go through every Pokemon game and use all of the sprites that they have, (which is most of them.) Or you could just use the sprite database in every Pokemon's entry on this site. Bulbapedia is also a great resource.
If you want MORE Pokemon sprites, I would download the famous fanmade Pokemon game, Pokemon Insurgence. They have all of the Gen 1-5 Pokemon sprites front, back, and shiny front and shiny back. They also have Gen 6 Pokemon with sprites, (including Megas,) and many fanmade Pokemon forms as sprites. (I never played Insurgence, but I downloaded it and I got thousands of sprites saved onto my phone and I had to delete them all.)
I hope this answers your question, and good luck!