yea it's pretty easy, go to Window>Animation and a tool bar thingo will appear down the bottom. Open up your first image you wan't animated, and in the very top right of that animation box, theres a lil arrow, click it and a drop down menu will appear, click on 'copy frame'. Change to your 2nd image you wan't animated, and click that same arrow and go 'paste frame'. You should then see 2 thumbnails of bouth your sigs, to time it just click where it says 0 secs and change it to however long you wan't it to stay on for...
hope that made sense