FIRST OF ALL I would like to point out this this is specifically targeting "thank you" posts, NOT "READ MY COMIC OK BYE" posts, the latter being the actual annoying type. I'll address it as quickly as I can.
I like to think of it as someone acknowledging that I've liked a creation of theirs in the first place and I personally would like a thank you image where I can show other people who visit my page what I like. If I didn't like it, I would have deleted the post/not followed the person!
It doesn't happen often, the thank yous, and when they do I'm like "Look, other people who come here, look at this thing I like! Like it too!" Because that helps me connect on this website with readers and creators. It's networking, if you don't do it, you're not going to get anything out there. I want my comics to help support me eventually, Yeah I do it for fun but hell. It's also starting to become a career choice and I'm not a doe-eyed kid who believes that if I believe in myself it'll just "work out".
That said: Hell yes, it's plugging. There should be nothing shameful about thanking someone and promoting yourself. There is something shameful about "READ MY COMIC IMAGE" when I already don't. and the person posting those doesn't read anything of mine, they just happened to see a comment box for them to violate. THAT to me is annoying and pushy. THAT to me is what the OP isn't addressing and I feel a little put off because the OP is talking about people who noticed you liked their thing. God forbid.
But, as a comic artist, if you don't find ways to plug and connect, you'll die out. And I hate to be the realist here, but if you think people shouldn't do this, and don't do this, then you also believe that a comic is something people find and share magically without ever being pointed into the direction of it. Ad if advertising happens through randomly finding a comic and then, over night, you're as popular as cucumber quest. No! You have to advertise your comic on someone elses' page as a thank you, you have to do it as an adspace, you have to network with other artists who will plug you. And if you think that other artists plugging you is "less shameful plugging" you might have a skewed look at things. It's no more or less shameless than plugging yourself.
There needs to be advertising. We have on-site ads here that share us with each other but so many people use ad blockers, so how else do we get ourselves out there? And if it's a thank you, then PLEASE, thank me and fellow artists with images. We want you to flourish, we want to flourish, and it's a form of quickie-networking.
I'm not going ot toss in any more, as the subject target seems to be wrong and upsetting to me. Thanking someone with an image is not shameless plugging, it's honest networking. Telling some random person to read your stuff is spamming, there's a clear difference as to why the person is on your page in the first place. Please think on that.