So you do have a Pictures folder. In that you also have Photos library and iPhoto libraries.
If you press the option/alt key down whilst starting Photos you will get the Choose Library dialogue where you can
select the library you want to use. For example you could choose the iPhoto library, then you could export it to a 'named' folder on your desktop.
Quit Photos. Start Photos again with the option/alt key pressed, Choose library, choose your Photos library, and once Photos has started you could import
the exported images from iPhoto from the folder you created on your desktop. Photos will now show both sets of images in the one library.
Alternatively you can go back to Snow Leopard, and keep iPhoto.
If you decide to upgrade to El Capitan again, make sure you are prepared, make sure your third party apps have updates available
for El C and that any peripherals (printers etc) are able to function in El C check for drivers etc.
You can make a genius appointment at your local Apple Retail Store to get them to help you out with
installing El Capitan.