There is no suggestion from the original photographers that spiders place water droplets on their heads. Web pages that re-post these photos don't have any references to support the notion.
There are at least two photographers who stage these photographs.
Consider Dmitriy Yoav Reinshtein whose gallery includes
- mantis with water droplet "hat"
- housefly with water droplet "hat"
- spider with water droplet "hat"
- spider on flat blue surface with artfully placed water droplets.
This last example shows that the photographer carefully places droplets to stage the photographs.
Another photographer is Uda Dennie who stages pictures with water droplets on the heads of jumping spiders. The droplets in Dennie's photos act as lenses to bring into focus backgrounds that consist of prints of other macro photographs.
These photos are real but highly staged. In particular, water droplets don't stay on the heads of jumping spiders for very long.