Your pictures aren't on' or 'in' any particular program. They're simply in a folder (or in a collection of folders) on your computer. They can be accessed from there by ANY suitable program.
First, locate your pictures. (Look in 'My Pictures' or go to Start > Search and hunt for the file name of one of your pictures. Alternatively hunt for *.jpg , which will find all pictures in the Jpeg format).
To open a picture in the default program (which appears to be Picasa on your computer), simply double-click on it.
To open a picture (on a 'one off' basis) in a different program, right-click on the file, select 'Open with' and then the program of your choice.
To change the default program which pictures open in, right-click on any picture file. Select 'Open with' but do NOT click on the relevant program. Instead, click on 'Choose Program'. Then select the program you want to use, put a tick alongside 'Always use this program to open this type of file' and click 'OK'.
Chris