There aren't a lot of ''megasets'' out there, and the ones there are are mostly either people who have scrounged up their own graphic sets or they ask you to pay for them...
The best thing you could do is go to a site like Sortitoutsi.net and look around yourself. I do suggest you stick to sets (leagues, nations etc.) instead of single clubs.
Sortitoutsi (susie) releases a new ''cut-out'' portrait megapack every year which they update frequently too.
Problem is though that you do not have a direct download link unless you donate to their site. So you are stuck with either using Rapidshare.com mirrors, or use torrents.
http://sortitoutsi.net/graphics/download_megapack/complete/1
Torrents would be the most speedy thing to use if you don't have RS. And around this time there are probably more seeders than ever with the Beta.
Now onto the installation instructions, which are quite easy.
All the graphics files you have downloaded go into your... ''C:\Users\username\Documents\Sports Interactive\Football Manager 2013'' folder
Inside that folder is a ''Graphics'' folder, if it isn't there you can create one yourself with that name.
You are free to create seperate folders inside if you wish to sort stuff, but you can just dump all your graphics files into that map and FM13 should be able to find it there.
Secondly you go into FM13 itself and often some files do not get loaded into the game, but there is a way to bypass that...
Click on Preferences -> Interface and in there you tick ''Reload Skin when confirming changes in preferences"
That is all, you have installed your badges/logos/portraits/backgrounds etc. Nothing to it.