How do I get rid of  tag in manually created block?
It's in fact because of the text input format. Going to admin/config/content/formats and removing the checkbox for

Convert line breaks into HTML (i.e. &lt;br&gt; and &lt;p&gt;)


for the input format you're using should do the trick (re-save the block, of course). Also, if you use WYSIWYG editor, it could do some formatting for you and put the image in &lt;p&gt; tag.