It depends on the complexity of the image. Images that are easy to compress can be rendered at a higher res than noisy images with a lot of color-changes and sharp contrast and complex shapes. But most images should do fine for at least up to 1280x1024 pixels. PSP has a preview option that shows the original image and the compressed one next to each other with filesizes already calculated. UFX also has an option like that and Corel Photopaint has it too. I think most popular packages do that.

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