Snapshots (photos, screencaptures) always are at the 'back' of the Skitch drawing space. The photos are not affected by the eraser. You can think of it like a photo which has an OHT (clear Over-Head-Transparency) placed on top. You can draw over all the photo ('Snap') by drawing on the OHT clear layer. If you decide to change what you have drawn with the pen, then you can do that without affecting the photo underneath.
To get some white space, press the 'wipe' button twice. The first click will always wipe any 'pen' off your image, and the second click will clear off any 'Snap' you have taken and will leave a plain white page. If you want a different colored background, then hold the Shift key while clicking on one of the colors on the right of the frame.
You can draw shapes, arrows, squiggles and text, and using the cursor tool move them around the page. Everything has its own 'layer' - that is, the first thing you draw will always stay on the bottom and the last thing you draw will be on top.
One exception is text, which always floats above everything else, to keep things neat.
The drawing features of Skitch may look like other 'fun' drawing programs (and Skitch IS fun!), with pens, fill buckets, erasers etc... However, there is something different about the drawing 'engine' underneath the hood of Skitch.
Skitch has a drawing engine which is based on bezier vector paths. "What does that mean?!!!", I hear you ask. Well, it means that the circles, squiggles and text that you draw are represented by some math equations, instead of the computer remembering what color each dot on the screen is. The benefits of this include:
Notice in this highly zoomed example the text in the background (from a
screenshot) is pixelated and chunky (bitmap), whereas the text at the
front and the pink shape is smooth (vector).
The Cursor
Great tip: At any time, no matter your current tool, you can grab objects by holding down the Command key
Pencil
If you hold down Option, the Pencil tool will turn into an Eyedropper, which lets you sample any color from the image, and set the drawing color to match
Straight Line
Holding down the Shift key lets you draw perfect horizontal, vertical or 45º lines. If you hold down Option, then you can draw polygons
Circle
Holding down Shift lets you draw perfect circles, and holding down Option makes the circle originate around where the cursor was initially clicked
Square
Holding Shift creates perfect squares and holding down Option makes the square originate around where the cursor was initially clicked
Paint Fill
Click inside an enclosed shape to fill it with your color
Eraser
Skitch has a really nice eraser - if you erase through a line, or
shape, then you are also 'cutting' the shape - which can then be moved
independently. Conversely, the paint bucket tool works like glue, and
will glue together any shapes that come in contact with the paint.
Text
Text in Skitch always floats on top, and is unaffected by the eraser.
The default text style in Skitch has been optimized for visibility. We've chosen specific colors, and fonts to really stand out above screenshots and photos. Text, by default, has a shadow and outline which automatically adjust color to the maximum visibility.
If you would like to set a specific font style, font size, and switch off the shadow and/or outline, then you can right-click on a word, or choose the Show Fonts choice in the Toolbox menu. A window will appear which will let you set the font you would like to use for this, and future text.
If you want to draw lines OVER text, then you could "drag me" the file to the desktop, and then drag it back to your Skitch area. This is what is commonly called 'flattening' the image, meaning that everything becomes one big image which you can draw over. Remember, if you do this, the resulting image will be like a fresh photo - you won't be able to edit any of the text, or move any of the shapes etc. Remember to save your image to History first before doing this.
Arrows
Skitch has really beautiful arrows that you can be proud of. Arrows can
be drawn in the horizontal or the vertical by holding down
the Shift key while you are drawing your arrow. When drawing many similar arrows near each other, it's
often better to simply copy (Option-drag with the cursor arrow) so that they are all at an
identical angle. You can change the default end of the arrow by going Skitch>Preferences and selecting Arrow Head at Start, or End.