Here is one quick and dirty way to add an image to the sidebar of a subreddit. You must be a moderator of a subreddit in order to edit its stylesheet.

1. Upload the image
2. Add this CSS to the stylesheet:
   /* add image to the bottom of the sidebar */
.side .usertext-body{ padding-bottom: 400px; background: url(%%image_name%%) no-repeat; background-position: bottom center; }
3. Replace %%image_name%% with the name of your uploaded image
4. Change padding-bottom: 400px; to a value in pixels that is at least the height of your image

Similarly, here is a CSS code that will add an image to the top of the reddit sidebar:

/* add image to the top of the sidebar */
.side { padding-top: 400px; background: url(%%image_name%%) no-repeat center; }


This adds a background image to the sidebar and positions it at the bottom. The padding creates enough blank space to make the image visible.

This won’t work if you already have a background image on the the sidebar as part of other visual changes on the subreddit.