Digital photos are data. Each image is built of number of data points called pixels and each pixel has a certain number of bits to describe the brightness and color. Images also usually employ some sort of data compression that can cause pixel values to be approximated.
Stretching a photo is a process called upsampling. The number of pixels is increased but there isn't any additional data so the values for the higher number of pixels are interpolated from the smaller picture which lowers its quality. There are tricks that can make upsampled pictures look better. Since upsampling blurs images the software can make the image look sharper; often it helps but not always and too much sharpening looks bad. Adding random noise or dithering can reduce the artifacts from the upsampling process. Since upsampling can wash out an image out pumping up the color saturation can make the image look better. While these enhansements can make the upsampled image look a bit better they do not actually add data to the image so they can only help so much.
well i understand how i can fix them, i always fix them to look like the original i would just like to know why it happened and if there was any way i could prevent it
Yes it happen to me too.When i attach my 132kb pic in yahoo mail it was 42kb.How could that happen.