You might want to ask over in /r/arthistory for survey texts, especially if there are time periods and places that interest you in particular. I'm a TA for an introductory art history course right now though, and we use "Janson's History of Art." It provides a pretty accessible overview of art in the Western world from cave paintings to the 21st century.

I'm currently in my second year of grad school studying art history, and when I was in my undergrad, my adviser told me to pick up as many art history survey textbooks as I could afford, and use them to study once a day for my competency exams during my first year of grad school.

In my opinion, Stockstad's Art History (5th Edition) is the best art history survey text. It's fairly comprehensive in most areas while remaining easy-to-read and interesting. Not to mention the older editions are fairly cheap (~$30) used.

Gardner's Art Through the Ages (15th Edition) is also a fairly good survey text, however I found it to be a bit remedial (it's intended to be used at a high school level) and lacking in depth in some areas, however it's generally pretty good in providing a general background in art history, which is what you seem to be looking for.

I do recommend you go ahead and take a survey course if your schedule permits it. Of course, I was a history major who took an art history course for similar reasons, and I ended up double-majoring in history and art history. Now I'm about to finish a PhD in art history. So that makes me a bit biased ;p

Any of the surveys texts mentioned by the other posters are fine, really. I think I prefer Stockstad, though I've never read Janson. You might also be interested in checking out Sylvan Barnet's A Short Guide to Writing About Art. That book will introduce you to some of the core concepts and processes that we art historians use in our writing that may differ somewhat from how historians work. Visual analysis being the big one.