The thing to remember here is that if you're not seeing big savings when compressing files it might mean the file has already been compressed. Read on for more details on each format.
If you compress a jpg that has already been compressed you probably won't see any reduction in file size, in fact, the file may ever be bigger!
If you compress a high-quality with very little compression, you're going to see great savings. This is when Squash really shines.
If you're still unhappy with the amount Squash has compressed your file, do the following: Try lowering the compression rate in the preferences, move it down in 10% increments, then compress the original image again. By doing this you can look at the image after each export and make sure you're happy with the look of the file and its size.
In our testing, we've found the savings to be made on PNG based images have been fairly small. This is because PNG is a lossless format, so often the best way to save on space is to cut down on a number of colours used. A flat colour png will always be pretty small.
If you don't need transparency in your image, we'd suggest converting the image to a JPG, as you'll probably be able to
You can always switch on the "PNG Compression" in the Preferences window, this will aim to compress the png a little more, but it's markedly slower.
So in short, only use PNG if the image contains lots of flat colour or you have some transparency in the file you need to preserve. Never save photos as PNG's unless you really need too.
Gif files are already often heavily compressed and optimized. Just like PNG's your savings here will vary depending on how heavily the GIF has already been compressed.
As a rule of thumb, the biggest compression savings to be made are always with high-quality JPG's.