Snapshots in OSX are natively PDF unless you're using 10.1 which did them in TIFF. You need to convert them to jpg or gif. Preferably in gif so it's smaller in size & doesn't over load the server. There are osx snapshot programs you can get where you can save in the format of your choice. Personally, I use Photoshop, resize & other adjustments, then ImageReady to convert to Gif format.

General rules about posting images: Keep them small in dimensions so people with smaller monitors such as 13-15 inch don't have to scroll to the right to see the whole image. Keep the file size small so people with dial up don't have to wait several mins for the page & image to load. Keep it small so the server doesn't have to cope with large images. This server is not especially fast. If necessary edit the images just to show what you want to show or is the most relevant part.

BTW Jpg is a lossy format, whereas Gif is arguably lossless. Giff has a limitation of 256 colours but you don't need that many colours for most images for samples. You can also use lossy filters to help reduce the size of Gifs. I've done sample images on the forum using as little as 4 & 8 colours which helped reduce file size.