HTTPaint Editor :

Left click + moving : Painting .
Right click+ moving : Paint switch on/off, useful for more precision on freehand drawing.
ALT + left click : Pipette.
Tool buttons : Press one to get one of the preseted tool. Any change on the parameters will be memorized while editing.
Preview window : The oblique line corresponds to the current tool parameters. You can try the tool in this window.
Palette : Change the tool color using either Red Green Blue (RGB) or Hue Saturation Brightness (HSB) coordinates.
Save : Save the edited picture on the server. it may last up to 20-30s, more or less depending on your internet connection. You can disconnect when painting and reconnect to save the picture.
Clear : Restore the picture as it was before launching the painting program.
Quit : Discard your painting, your picture is ugly.
Undo/Redo : Switch between a temporary backup picture and the current one.
AutoConfirm : When checked, this performs a backup of your picture every time you release a mouse button, else you have to click on the valid button.
Diameter scrollbar : Adjust the diameter of the stroke.
Pressure scrollbar : Adjust the transparency of the stroke.
Fading scrollbard : Linearity of the stroke shading outside the center zone.
Frequency scrollbar : The distance between two strokes when you're painting. Use (1-4) values to get a continuous line.
Center : Diameter (%) of the zone where the shading is not applied. Useful to make a large antialised stroke.
Noise : Randomize the pixels. Try (noise=3,pressure=0) then (noise=3,pressure=10).