Can't change logo due Can't upload images

I am at the bluehost, bought template from Mojo, installed set the demo page as default which I am thrying to reedit to my own desighn… and now can’t even upload a Logo, images or whatever. I got error every time: “An error occurred in the upload. Please try again later.” I noticed Logo images are on your server, probably because of last, but how do I change that.