How to change image size

Sky Land
From the pictures you can't just change the extension. Need to resave it in another format. As a result, depending on the algorithm can be loss of information when recoding.
Is there a good program is PaintNet.

Sky Land wrote:
Tell me how you can change the extension of the image?
And the title of the topic - how to resize pictures. Different things $)
However, both changing it in any image editor, whether peint or photoshop.