the point is that I've to insert data into an textfield
if i use camtasia ive to work with several screenshots per screen
I would also spend some time checking out the tutorials that Andrew Devis has on the CreativeCow site. There are currently, just over 40 of these AE Basics tutorials. A few of them deal with Text, and a few with stills etc. Spend some time on the site, and I am sure you will find some tutorials that will assist you.
AE has text layers. That aside, Camtasia records video, not screenshots. Camtasia can also store live mouse coordinates in a metadata stream and attach any kind of cursor graphic you like. Camtasia can insert text as well as "callouts". Need I go on? Point in case: You are probably looking at the wrong tool trying to do this in AE. And even when you do, explore all options before writing it off. You really need to watch those basic tutorials and adapt your thinking rather than settling on a fixed workflow.