Is it possible to make a text or image appear for only a few seconds?

Yes, it is. The way to do it is to either place them as a part of a field, or to cover them using a field.

Then you can use the setTimeOut method of the app object to show or hide that field after X amount of time.

Here's a simple example. Let's say you have text that you want to show for 5 seconds. Create a (read-only) hidden text field on top of that text with a white fill. Let's call it "CoverText1". Now, under your event you enter this code:

var to1 = app.setTimeOut("this.getField(\"CoverText1\").display = display.visible;", 5000);

This will cause the field to become visible after 5 seconds, thereby covering the underlying text beneath it.