​
i know on
$1 image search you can specify public domain and that will probably be pretty helpful to avoid take any kind of takedown notices due to copyright.
i did a few searches, "happy background" was one i tried but duckduckgo seems to provide happy birthday related crap and isn't as good as the results i got through
$1 but yandex doesn't seem to let you specifiy a license sooo i guess although the ai is better there it probably isnt as helpful.
i can't really speak to the accessibility of those sites unfortunately.