If you’re going to repost/reblog someone else’s art, make absolutely certain they have given permission to post that specific piece of art. (It’s obviously OK if they posted it themselves)
I do not care if you think it’s alright if there’s a source, unless they have written on their profile their own rules for reposting. Posters, please DO note if you have gotten permission.
Can’t find rules, then ask. Language barrier isn’t a big deal. (I would know) Manners come before this.
If their art is elsewhere and they do not know, that’s extremely rude. That’s their work. Not yours. They deserve to know, period.
You have no reason to fear rejection. If you fear it, you’re too lazy, or can’t find the original artist, then it still doesn’t give you the OK to repost.
This goes for pixiv, personal websites, twitter… pretty much everything.