To use a picture from your computer as your background, upload it to a free image hosting site. The site will give you the image's URL after uploading. Paste that URL into your Tumblr HTML code as ...

Open an image file you want to use as your background. For best results, choose a high resolution image between 1800 and 2000 pixels wide and up to 2200 pixels high.

The background on this slide is an image. The easiest way to customize it is to select a different image. 1. From the Design tab, click Format Background. 2.