Hi Zoe,
Once you've found the letter I clue you'll be able to pick up the similarly-themed paintings.
For the letter I, locate two paintings next to each other which are the same. Click on both paintings and Holmes will comment that they are identical. Now use your magnifying glass on both. Scan all over the painting on the left until you see a tiny Lupin standing on a balcony, and left-click. Then use the lamp on that painting to see more clearly.
