Featured Post Slider Questions

I am using Catch Evolution theme - version 1.2 - and the Post Slider has two photos. One is a beautiful white flower, which I like. And one is a brick building, which I don't. How can I remove that brick photo but keep the white flower? And what are the exact dimensions needed for MY photos to put in the slider? I tried adding one, and it also included text from my post, so perhaps the photo was too small? Rather than experiment with sizes, can you give me a number? Thanks!