Ever need to know exactly how many pixels something on your screen is, but you don't want to open the tool to measure it - or don't know what that tool is? Viola. Listen to this:

In this image, I took a screen shot of the spot the image needs to fit on my site, seen in the
upper left side and looked at it's size 579 pixels wide, then resized Dirty Bird's portrait to fit
exactly in the space I saw on my site: 579px wide by whatever height is proportional.

In Mac (I would search Tucows for an equivalent screen shot program). Use the screen shot function with the crosshair at COMMAND>SHIFT 4.

Go to the area you want to measure, click, hold down and drag across your selection to complete a screen shot of the area.

Go to the file, which lands on my desktop and look at Get Info or Right Click for Properties in PC to get the exact pixel size -

Viola, your area is measured exactly how you see it on screen.