The new(ish) funnel image optimisation feature renders png images with transparency in them without the transparency. This looks terrible.
The workaround is to manually go in and disable this feature on every single funnel individually. When using images on snapshots, this means there are potentially hundreds/thousands+ of funnels that this needs to happen on.
I believe this is a bug and should be addressed quickly. Support have confirmed that this is a known issue but there is no ETA a fix. They recommended adding an idea here - so here it is.