Image without wrapper in forum sidebar

Hi, I'm trying to add a block with an image in the sidebar without any wrapper, as you can do in vbadvanced:



I've made a block with a custom template (block_html1) but I can't remove all the borders of the block, it always stay at least one, and the result is not good. This is what I want:



I've tried to insert the code manually modifying files, but I can't find the file where sidebar is.

Any help will be appreciated. Thanks!