I have strange digital image

Hey Tom; this always hurts my brain, but here goes. Your image file is 6mb. That's the easy part. When it is saved in the .jpg format, it is "compressed" in order to store more images on a memory card or hard drive or CD etc. . Otherwise the sheer size of digital images would only allow a few on a memory card before it was full. Your 1.8 mb file size is the compressed size. When you open the file it is uncompressed to its proper dimensions. The amount of compression is usually chosen to optimize its use. For instance Picasa has to store so many images, it may compress them more than another application. Each time you open, make any changes, and re-save the same image, a small amount of data is lost or destroyed (that may explain why they open up to 5mb in Photopaint). The less compression used, the more "true" the image remains. Most cameras compress the images to about 1/4 their expanded size. That's how a 1gb memory card can hold hundreds of 6mp images, when it could only hold about 170 if they were uncompressed. I hope that helps a little.