How do I limit the maximum height of an image in signatures?
Under 
Home >> Administration >> Site Configuration 
Go to Signatures for Forums
Inside template for signature you can set the height inside the div as follows :

&lt;div style= "height : 100px;"&gt;%s&lt;/div&gt;


Home >> Administer >> User Management 

Go to Picture Support option and enable it 

After that you will find a field Picture maximum dimensions: where you can set your image maximum height and width
 check the image link