You can get images into Skitch in various ways. These ways include:
The 'crosshairs' method of Snapping is the handiest, so you will probably use that the most. Click on the Snap button and then click and drag over the area you want to capture. The other options are available from the Toolbox menu.
Often you will want to take a Snapshot of an entire applications window, for making a tutorial for example. You can do this by using the crosshair Snap, and simply clicking on a window (Rather than click-and-dragging). This takes an auto-cutout Snapshot of the window you click on, with the contents which are currently shown in that window. (Skitch cannot currently show all scrollable contents of a browser window, for example a long page that doesn't fit on screen). The resulting image will be tightly cropped to the image, but due to round OS X windows and shadows, there will always be some background viewable. In the Skitch Preferences you can find options to present a border of space and an optional shadow around the image automatically, which can save you oodles of time.
With a white background it's easy to take a picture of a window, for placing on a white page.
A transparent background makes it very easy to use the images in professional image layout programs. Note: Exporting the image in .jpg will render the transparency white.
The 'Desktop' option is ultra handy, because it fills the edges of the shot with a clean view of your desktop below. This means you can have perfect results every time, even if you have windows underneath your Snap target window, and files and folders all over your desktop. You just need to click on the window, and Skitch will do the rest for you.
The full screenshot captures everything in your display. You can find this in the Toolbox menu.
The 'Frame' Snapshot is handy for producing a
series of Snapshots which are all the same dimensions — You can
find this in the Toolbox menu on the frame. It will make the center of the Skitch frame see-through, allowing you to 'frame' your Snap.
If you want to override the timed count down, then press Shift and clip Snap when viewing your preview.
You can drag most image files types directly on to the Skitch window from the Finder, or any other application. You can also Copy and Paste into Skitch from other applications.
The Photos button takes you to an image browser for images in your iPhoto library, Aperture library, and Pictures folder. You can double-click on an image to open it in Skitch.
When you resize using the Skitch corner handles, you are affecting the
size that Skitch will output. That is, the
size that you see equals the size that the other person will get. There
is one exception, and that is if you drag or Snap a really large image
with Skitch, then Skitch will automatically resize the image, and offer
an option for "Export at original size ". If you check this box then the size
of the picture from Skitch will be identical to the original, pixel for pixel.
You might use this feature if: