I'm using SimpleNote on my Mac and iPhone for pretty much all my note-taking. It automatically syncs notes between computers/devices, so might not be for you if you only want local notes. I find it's really convenient for accessing my notes everywhere, though.
It doesn't look like big sticky notes, instead you get a list view of your notes with a preview, from which you can open the entire note. I find it really pretty, but it might not be the aestethic you're after.
Edit: Forgot to mention that I use the DashNote widget on my Macs. There are other options as well, but they mainly come as stand-alone programs.
Best Answer
You can open an image in Safari (even from your disk, by drag'n'drop) and click on
File
->Open in Dashboard...
.Check the result and if it's not satisfying, resize the image and redo the operation.