In your code the UserData of the root object are overwritten 119 times:
for i = 1:120
imgs{i} = imread( sprintf('%d.jpg',idx(i) ));
set(0,'userdata',imgs{i})
end
Better use:
for i = 1:120
imgs{i} = imread( sprintf('%d.jpg',idx(i) ));
end
set(0,'userdata',imgs)
Now the cell imgs is stored in the UserData.
Btw., the Userdata of the root object are equivalent to using global variables. So einer rely on globals directly or consider the serious warnings about globals and avoid this dirty trick. Better store the data in the UserData or ApplicationData of a GUI instead of the root object.
