Not trying to come off as rude, but that's your own fault for not reading the rules, which are clearly posted. Signatures cannot exceed 350x650. There's plenty of online tutorials on how to crop or otherwise resize a photo, and it can be done via photoshop when you save it.
Click File and go down to "Save for web"
Save for web window opens. On the bottom right, there's blocks that state "Image Size"
In the image size boxes, you enter the desired dimensions of the image
From there, you click SAVE, then save your file and upload it to your image host of choice.