secret image, margin, image description

Hei hei!
I have three (I hope very simple) questions, I can not solve.
1. In every post a have an image I don’t know where it comes from. Here is the code:

<p class="entry-ad clear hidden-print single-post-ads-between center-block text-center"><a href="#" target="_blank"><img src="http://placehold.it/300x250" /></a></p>

I don’t know how to delete it from my posts.

2. margin
I would like to change the space of my posts, (and also where the posts are listed under the slider) and the sidebar. I mean that the text and the sidebar move a bit further to the end of the website.
So I have more space and can upload bigger images (can I?)
Sorry for my bad explaining, I hope you know what I mean.

3. the image description
How can I delete the frame of the image description?

I hope you can understand my shitty explanations 😀 and can help me! 🙂
Thank you!