Generate Random Image Name

The final images saved to `$TMP/cgc/horizontal/` should have randomly generated names instead of using sequential numbers.

Before:

* 2.jpg
* 4.jpg
* 6.jpg

After (example):

* 6b8da498-64e6-497c-b2b6-d441a538c2fd.jpg
* 5fa37ff8-0ada-4710-a721-1192a2728cf9.jpg
* 350b28c9-656f-464a-b2d7-5ba8df343c28.jpg