Please send WordPress admin login details of your site at firstname.lastname@example.org
so we can check your website and solve the problem regarding forums sidebar.
If you want to reduce the height of slider image, please use the image of that sizes.
Please note that all images should be of same sizes for proper display of homepage slider.
If you don’t want to use this default image slider, you can use any image slider WordPress plugin
Go to Appearance » Theme Options » Homepage Slider
Enter your slider shortcode which is provided by plugin without php tag