Plates (illustration pages) can be attached to a text block in several ways.
Some plates are printed on folded sheets that are sewn into the text block as part of a signature. Other plates are printed on shiny, stiff paper, then tipped onto the page with glue. As the original glue dries up, the plate can separate from the text and fall out of the book.
Plates can be replaced by re-tipping them to the page or reattached by creating a Japanese tissue hinge between the plate and the text block page.
Use the technique for hinging in a text page to hinge a plate into a book.
Check the original placement of the plate on the page as some plates are smaller than the pages of the book and are often centered on the page.
If the plate was originally tipped in along the side (usually the left side of the plate) it can be reattached along that edge or along the top edge of the plate.
Hinging from the top edge of the plate gives more support to a heavy plate.