Hi Mark,
What snipping tool are you using? When I use the built-in screen grabbing tools on my mac (i.e. shift-cmd-4 to capture a rectangular selection) it outputs a png file. If you are using a third-party tool that only exports TIFF, frankly, I would get another tool.
If you are using the Preview application to convert (the command is actually Export), then make sure you are setting the quality correctly. You can safely set the jpg quality to 90+ percent. The upload feature on the web site does some optimization to the size and compression of the images to make sure they don’t adversely impact the performance of the site.
B