slides deteriorate with age, and the image quality degrades.
When I taught AP art history I spent the year prior going online capturing images that I wanted and made 34 Power Points that cover everything from the Caves to the Moderns. The images that you can capture from websites (especially the museum ones, but also many other websites including professors) are much crisper and truer to the colors of the art work than any slides would be able to "keep". Yes, it took time, and I spent that year every night after school until 1:00 am doing it. When I retired I passed the DVD's on to the next person teaching the class. He will be overwhelmed enough with handling the material for that class without having to worry where he will get the images.
I would recommend that you only scan those slides that you know you won't be able to get the images from online. But I do know for you, time is of the essence and you don't have the full year to prepare.