Could this be due to the photos being assigned to open in Irfanview ?
If so do the following.
to get your images to open in Picasa, right click any of your photo thumbnails, in the box that opens select Picasa. and tick the box at the bottom which says "Always use the selected program to open this kind of file".
If Picasa is not displayed in the list click on the browse button, then browse for and click on Picasa, another box will open, select Picasa. exe. Not forgetting to select "Always use the selected program to open this kind of file"