Here are some possible translations depending on the context:
* Picture: llun (pronounced "lloon")
* Photo: llun or llun ffotograffig (pronounced "lloon ffotograffig")
* Snapshot: llun cyflym (pronounced "lloon kyflum")
* Informal picture: delwedd (pronounced "llewedd")
The best word to use depends on the specific context and the type of picture you're referring to.