This is because in your config you have startfolder=content/, which basically is a folder with no images inside. You have two options:
#1 - Set the startpath to one of your actual gallery folders. Then it will go directly to a gallery on start-up. (To do this go to admin -> config -> startpath --- point it to one of your gallery folders).
#2 - If you just want a single start image, or if you want to use one of the filemods(randomimage, photoblinds or slideshow) -> Create a new folder, set "use as[filemod]" in right panel. Then follow instructions here depending on what you want:
https://www.photo.gallery/v2docs/articles/filemod/
Then of course, you need to do the same as in #1, and point your startpath to the new folder.