Sure! I checked your custom CSS just now, and I found this:
background: url (hthttp://wimbledoneschool.files.wordpress.com/2014/07/logo-final2.jpg) left center no-repeat;
Btw, including specific examples such as the CSS you tried is always really really helpful.
I see a couple mistakes in your CSS. First, it begins with "hthttp" but it should be "http" — looks like just a typo there. Next, there is a space between "url" and "(" and there shouldn't be a space there (CSS is very picky about spaces!). You forgot to add the "min-height" value. Finally, it looks like the image you referenced in your CSS is 304 x 341 pixels, and that seems too big:
WordPress.com has a built in way to resize images. (We are clever like that.) :) And you can trigger it using a parameter on the image URL. For example, if you wanted to use the image you already uploaded but make it 120px wide, you would use this:
Here is an adjusted CSS example using that image URL and with the syntax fixed:
background: url(http://wimbledoneschool.files.wordpress.com/2014/07/logo-final2.jpg?w=120) left center no-repeat;