Inspiration vs Plagerism

I had an interesting wake up call this morning - it came in the form of a reply, which I probably misinterpreted, to a comment I posted on Facebook. There was a quote posted which I identified with and made a very rare comment on - that I liked it and could see myself using it in some form or another. The response was 'Thou shall not steal from the internet' and I went totally apeshit!
Not being a big user of Facebook, I didn't look carefully enough to see that 'LIKE' was attached, along with a smiley face, and some more wording '...That is my job' so I'm hoping it was done tongue in cheek. But I was really agitated for an hour or so. It felt like I was being accused of plagerism, or at least thinking about commiting the offence.
I don't plagerise, but I do use quotes, always attributing to the original source. Shakespeare and Robert Burns are my favourites to draw on. I had both drummed into me at school, so can reference them pretty easily. And they do come up with some cracking lines!
But the situation got me thinking... is there a fine line between plagerism and inspiration. If I were to use the quote that was posted on Facebook then I would paraphrase the words but keep the same sentiment. Is that plagerism, or using something someone has writen to inspire my own work?
Why this is so topical for me at the moment is because I have started to use Tumblr as a means to advertise my work (see earlier blog). I do this under the title 'Jack's Inspiration' hence my questioning myself. Bear in mind it is an advert, not a retrospective. What I do is find an image that fits something I have writen - post the image and the extract from the book with a link to the book page on this website. For example, the guy above was used yesterday along with an extract from 'Nabbed Again'. But I found him after I wrote it, so not guilty there.
However!
There have been instances where I have seen an image and used it as genuine inspiration. Maybe the way a guy looks, or some filthy act that is taking place, and then write the scene with this in mind. Can that be a form of plagerism? I hope not. But I will certainly think a little more carefully about what I post in future. It is so easy to point fingers, and so easy to make a mistake without intending to play foul.
Anyway, I'm glad to have got that off my chest... Now time to do some creative writing - 'The Wild Side of Christmas' awaits, and there is a pretty obvious deadline that needs to be met.

Jack