Transparent Overlays in C?

[Wolfgang Meyerle]

Hello,

currently I'm working on a call monitor applet which shows me the phone
numbers of the incoming calls on my display via osd.
For that I'm using the libxosd for now. But I want to overlay more
phantastic things via OSD like a photo of the caller or sth like that.
My question is now where can i start off first. Which functions could I
use for that implementation. I want to use functions that Xorg provides.
Has anybody done this before?

Greetings

Wolfgang Meyerle,
Informatic Student