There isn't a way to get anything taller in your signature, but if you want a wider picture you could split the image into pieces and put them in a line - you'll probably need to put them in link tags to get them to join together.

The only way to bypass the signature limits would be to simply paste the image at the end of every one of your posts and not use the signature space. Although that negates the advantage of an editable signature function that allows updates to display on all your posts - past and present.

If you just need image editing software to resize a picture, try IrfanView, Paint.NET, or GIMP.

I'm not sure exactly what you're asking. If this response doesn't cover it, reply to explain further and please include an example image.