I'm creating images for banners and the landing page. What is the recommended Width and length of the image?
It depends on your theme. Here's the tutorial for the Serenity theme and the widget it shows mentions the dimension: https://community.socialengine.com/blogs/597/70/serenity-theme
As our Learning Center linked above now has most of the tutorials, you may want to search for answers there first.
https://community.socialengine.com/blogs/597/67/banner-manager this tutorial doesn't mention size. The best thing to do is to use inspect element in your browser and check the dimensions of the widget on your site as there are various places you can put the banners.
Please remember to state your version of SEPHP as v6 has a different theme and features. In case you are using v6, there's a tutorial for Elpis almost done being imported. The theme settings are basically the same as the Serenity theme. https://demo.socialengine.com/admin/content there you can see the widget for the banner. When you click to edit it, it shows the input box for the height. As these are responsive too, the width would depend on screensize.
Thank you Donna