A quick way on the Mac to get a file up or reduced in size is to take a screen shot of your picture from your screen.
Hold the shift key, command key and the number 4 all at the same time. There will be a cross hair symbol now where your curser was. Put that in the upper left hand corner of your image. Hold the mouse down and drag a box over what you want to capture. That image will be dropped on your desktop as a PNG file. It will say screenshot( date and time ).png
I have good luck with using that technique to get stuff on the forum quickly. Depending on how big your mac screen is, you can reduce the size you see on your screen before taking the screen shot-
Hope that helps
Jeff-