There are two different verbs at play here:
- Overlie; past overlay, past participle overlain.
- Overlay; past overlaid, past participle overlaid.
They have overlapping meanings, but it is overlay that is used of superimposed images. It is also overlay that exists as a noun for such a superimposed or covering item.
Hence, you want overlaid.
Better still, just use the plural noun overlays (all the more so, since the components are overlaid while stored in separate files, but overlay the noun does include things that are intended to be overlaid, as well as things that currently are overlaid, and the term has an established use in terms of superimposed computer images).