If you want the flexibility to really create sigs from scratch, you'll need an image application like photoshop, paintshop pro, ...
The quickie ones have been the shrink, crop, resize versions. You can use that IRFan tool to do all that. If your source is big enough and the core content can be "shaped" into the right dimensions, IRFan will work for you. It just takes some trial and error. Sometimes the dimensions won't come out exact but as long as you are within the 450x100, you'll be OK. Sometimes, it will be 440x98 or 300x100. As long as it fits in the guideline, you are OK.
IRFan can resize your image while keeping the same aspect ratio, or stretch and squeeze the pic. It will just look weird doing it the second way. Once you get to a decent starting size, you can use the select tool to select what you want to crop. This select tool will tell you the dimension od your selection so you can tweak the selection and move the selection box around until you get a good selection to crop.
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