I always use PicMonkey to resize photos. It's very easy to use and I can resize my photos as specific as I want.
To add/change a signature photo, click on 'User CP' in the top left corner. On the left-hand toolbar, under 'Settings & Options', click on 'Edit Signature'. There you can customise the words in your signature, or below it, attach a signature photo.
If you can't make your own signature, you can request one
here. You may have to wait a while, but it is so worth it.