If you use

$image = GD::Image->new($filename)
[download]

It will work out what type of image it is for itself--modulo those types it supports. A quick look at the docs didn't reveal any mention of .tiff though. .png is my preference anyway, as it compresses very well without any artifacts or side effects other than taking a few more milliseconds to load.

By way of example:

• A 66 MB 6600x3400 24-bit tiff containing 1565866 colours, which takes 1/3rd of a second to load.
• Saved as a 24-bit .png with maximum compression reduces to 44 MB.

  Still contains 1565866 colours and shows no signs of of any artifacts even at 500% magnification. but it takes 3 seconds to load (and decompress).

• Saved as a 24-bit .jpg at 'best' quality, it reduces to 20 MB.

  But now only contains 650125 colours, shows clear signs of the lossy algorithm even at modest magnifications and still takes 2 seconds to load (and decompress).