Info Visualization – Custom design for many items over a period of time

I want to show a sequence of user events (such as logon events) in a chart. This would normally contain zero to infinite entries.

To represent users, I intend to use small icons of, for example, about 64×64. But how can I portray them with good UX over a timeline? For example, if I simply drag them over a line that represents time, blanks and overlaps can occur. When I play them in a fixed spatial order, I lose the information about how much time passes between the events, and may still need to reclassify them. I am also worried about efficient use of the space screen, which is not possible with a series of small pictures.

What is the most elegant design for it? I'm open to any suggestions, including multi-page fragmentation, with interactivity like sliders, one-click pop-ups, and so on. It is important to be able to access all events, time and user, and efficiently search the time axis when searching for a particular time or symbol.