I don't know of a website that already has comparisons, I used to make my own using artchive.com, and copying and pasting images into a powerpoint that I showed. I would recommend doing the legwork yourself because you can then pick images you are familiar with and can speak about with some confidence.
As for art activities with writing, I taught an art history course and so here are some exercises from my bag of tricks. Many of these tricks were picked up from workshops along the way.
1. In envelopes put sticky notes withe elements and principles on them. Each kid gets an envelope with a specific thing, and then when you show an image, they have to come up and stick their stick note onto an example of their element/principle.
2. In envelopes put a type written sentence that is a fact about something in the painting, about the artist, or about the times. Each kid gets an envelope. Then start talking about the painting. Encourage the students to interrupt politely (raising hands works) when they have something relevant to add from their envelope.
3. Take 20 images you have already looked at with your kids (this is an activity you do at the end). Pin one each to the BACK of your students. They must go around and ask 20 questions to figure out what image they have pinned to their back.
4.Divide students into groups. Each group is color coded, i.e. the blue group, the red group and each member in the group is given a marker the color of their group.
Disperse images around the room, by hanging them up on a larger piece of paper. So you could take a postcard of a famous painting and tape it to a 18 X 24 piece of paper. Students in their groups go from image to image and in a timed manner (like 3 mins) they must as a group write down as much on the big piece of paper about the image that they can (i.e. elements, principles).
5. Divide class in half. Play pictionary on the board using art terms one team against another.
6. Put up an image, and have students write a letter to a friend imagining they were in the environment of the painting and also describing the painting. A sort of "wish you were here postcard" thing.
7. Give each student a notecard. They must write one word to describe the painting. Then as a class, they order the cards (on the floor) starting with the most important or strongest word first. They then take the list of words and write a piece of poetry using those words to describe the work of art.