for this bug : "Picture pictures\overview.paa not found at startup when joining server despite the picture is present in the mission folder, I don't know how to fix this."
in fact, error message present only on first time you launch mission, after server have charged mission if you restart mission error not longer present and picture displayed correctly.
I think this is an internal error of ArmA, only original "overview.paa" picture present in ArmA3 game will be loaded correctly on first time, all "overview.paa" picture of mission made by community have same problem on my server.
if Somebody know how to force server to read content of missions present on server, maybe that can help us to resolve this problem.