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Pixel clubs are online pixel art projects were various people draw something, usually off a tempalte procided by the pixel club creator, that fits the theme of the pixel club. Whoever creates submissions for pixel clubs may submit them to an official list of members, but some might not submit or the list may not get updated so if you wanted to find every image created for a pixel club you'll need to go searching Neocities and other personal websites. The images work like adoptables where you always credit and link back to who made it, and it's up to you whether you want to display images other people have made for the club on your site and which ones you want to share on your site (perhaps only the ones that fit your aesthetic, your favourites, ones of media you have read/watched/listened too, etc). I have a list of pixel clubs you can take part in on my resources page.
"Adoptables are essentially a collection of pixel art you can download and put on your own website, or digital art that you offer to others to download and display on their sites. Adoptables, which are linked to the creator directly when placed on your website, can range from animals and toys to original characters and inanimate objects at any size. Typically, visitors can see them displayed on a "toybox" page on your website, but you can also have any type of page for collecting things from other websites, put them on your index page, or wherever you'd like. If you create a page full of original art for others to adopt, be sure to tell them to link back to you! And, for others, be sure to read the pages to make sure, when downloading adoptables, that you're downloading from the website of the original creator so you can properly link back." [source]