You shouldn't be using fanart that doesn't belong to you and that you don't have permission to use. Just saying that it doesn't belong to you isn't good enough and is still art theft.
First of all, thank you for wording your message politely. Secondly, with my blog being a couple of months away from its one year anniversary, I’m honestly surprised you are the first anon to bring this issue up. I have been contemplating this idea for quite a while now, and still don’t know how to really properly answer this. But I’ll start off with this:
Yes, what I do is what you said, art theft. Plain and simple. And I’m not going to excuse myself with the “bandwagon” claim that every icon maker is doing this, because yes, a lot of icon makers are doing this, but at the same time there are those who do not. I try not to when I can, especially when deviating from the Hetalia fandom, but I still do. Why? Because I do feel a sense of guilt. I understand the effort and work artists put into their works. Being an artists and writer myself, I understand the blood, sweat, and tears that go into making fanart/fanfic. At the same time, I was never really bothered when someone else reposted a work of mine, or translated a fic in another language. I was only ever bothered if anyone tried to claim them as their own. That to me was unforgivable.
However, I understand not all artists share my opinion, and so when I started icon making a few years ago, I always said the artwork was not mine, and that if the artist didn’t want their work made into an icon then they could contact me and I’d remove it. That’s still true for this blog. If an artist didn’t like what I did to their work, then yes, I would immediately take it down. It’s not my right to keep it up when it’s their work. Even if it wasn’t the artist, but someone who knew the artist didn’t like it, I would still remove it. At the same time, I think, these are icons. They are used for harmless things like our tumblr icons, or roleplaying threads. There’s no malicious intent, nor is there anyone claiming that the art is their own. Instead it makes people happy, especially when they can find their character.
Nonetheless, I understand your point, and therefore will ask this: Should I remove all icons made of fan art from the blog, or should I wait until an artist tells me to? If it is the former, then I’ll do it. If it’s the latter, then I will keep the blog the same.
Thank you anon for bringing up an issue that is often ignored.