A simple example of changing the alpha of a sprite.

Sprite Alpha using Canvas

ctx.save();

// 50% opacity
// Anything drawn after this call (but before call .restore) will be drawn at 50% opacity
ctx.globalAlpha = 0.5; 

ctx.drawImage(
	spriteSourceImage, 
	sourceX, sourceY, sourceWidth, sourceHeight, 
	destX, destY, destWidth, destHeight
);

ctx.restore();