on that log appears user name? is it a valid user name?
If it is a valid user name, maybe someone tried to logon and mistype the password? i can hapen sometimes (even you can type wrong).
Before starting thinking about hacking, pls verify if it wasnt just a normal usage, but a user/password mystype.
Even is not the case, from time to time some guys will reach your site and try something. Its a matter of time. Dont worry, except they are getting close or you have a weak security schema.
If you know nothing (or near nothing) about IT security, i strong suggest you to talk to someone that knows that. Although your site doesnt appears to be "critical", i dont think that you want that someday your home page appears defaced with a picture of someone naked.
You (or a expert friend) can make your site "secure", doing just some basic customizations.