There are two simple steps: capturing the image and uploading the image. This is one of the little features that I'm sure Mac folks take for granted but that as a Windows guy I find so helpful.
Capturing the Image
On Windows capturing an image can be a bit of a challenge. If you are looking at a screen shot you want you can hit Print Screen, which will capture the entire desktop and open windows. As an alternative you can hit Alt-Print Screen to grab the current window. It makes a copy of the area and places it in the Windows clipboard. You then need to paste it into the appropriate location.
Everyone on Windows has a copy of Paint, a simple painting program. I would fire that up, paste the contents of the clipboard into it, then use Save As to make the image a JPEG and place it somewhere I can get to quickly.
If I just want to grab a copy of an image that already exists on a corporate web site - say a logo that I can put into my blog - I right click on the image and use the Save As functionality.
On Mac it's much easier. If the item I want to use is a graphic on a web page - say the logo of a product I'm writing about - I simply drag it out of Safari and onto my desktop. Done.
I can also use Mac's Grab utility to suck out portions of the screen. I can grab a window, selection, the entire screen or a timed screen - which let's me get out of the Grab menu and set up my environment the way I want it captured.
I use the Command-Shift-4 shortcut on my Mac to just grab a selection of what I am looking at. It drops a PNG formatted image right on my desktop.
Uploading the Image
In the Windows world I upload my image into Blogger (the site I use to manage this blog) by clicking on the Upload Image button. I am then prompted with a dialog that lets me choose the image I want to upload. Usually the image is stored in a different path than the default that I get when I click the Select File button - I have to navigate to it.
Most web sites employ a similar model; you can choose a file by clicking the Select or Choose button or you can type in the path to the file and click an Upload button. This uses standard HTML controls to handle it.
On my Mac it's pretty simple. Since I keep my captured images on my desktop I simply drag the desktop image onto the Select File button and it knows which file I mean.
While not a huge time saver it's little things like this - the implied dragging and dropping - that make the Mac interface so cool. I actually get a little pissed off when I try to drag something and it's not drag enabled and I think it should be.