UMAP: Placemarks with photos in the pop-up

I found out how to link pictures to placemarks, that show when you click on them, which is really nice.
The syntax for linking a picture with a custom width in Pixels is as following:


But I highly suspect, that there is a rather lox maximum of pixels one can specify to be shown, and thus the up-popping photos appear to be really small, like 300 Pixels or something. But I could nowhere find a max value, or a way to show larger photos there.
Anyone knows more maybe?