Yes, there are artists with single images for sale. I should also note that there are many clip-art collections for sale as well.
As has been mentioned, these are usually more detailed/involved than a piece of rope, and aren't as use-specific.
For the record, I have purchased nearly every clip-art collection. I have not purchased ANY of the single pieces. First of all, I think they are too expensive (why pay $10 for one image when I can pay $5 for 20 images?) and second (and probably more importantly), I find the usage rights granted on the single-shot pieces to be (WAY) too restrictive.
*shrugs* I know it takes time and talent to create art - and that's what is being paid for - but considering that I can commission a quarter-page for $20-40 and get exclusive rights to use it (rather than being just "one more person using that image") AND get it done to my own specifications, I find a piece of spot art (as the rope is) that is not exclusively licensed to be worth considerably less. The single images you mentioned are not attractive for the same reasons - for the cost of 2 to 4 of these images, which may or may not be quite what I'm looking for and with a lot of restrictions on my use and the chance that they'll appear elsewhere as well, I can go commission one of my own with NO restrictions on use and that is exactly what I'm looking for.
Not trying to bash or be rude - it's a nice piece - but try to consider things from the potential buyer's perspective as well (I don't think the other "single piece" listings have done that, and given that I haven't seen them even on the top 100 non-RPG things on RPGNow, I'm guessing they're not seeing much in the way of sales, though I could be wrong).
--The Sigil