To convert images of scores from third party found in the net (e.g. BSB) to high resolution monochrome images, it is a good idea to use an image converter such as IrfanView (available for free download). This software offers batch conversion, which means you can convert a large number of images automatically with the same parameters. Instead of resizing the single pixels independently, it uses a Lanczos filter, which results in better resolution (instead of just bigger pixels).
How to work with IrfanView
In the 'File' menu, open 'Batch Conversion/Rename...'
Here you can choose the folder which contains your original score images and also one where the converted output will be saved.
Use the image settings window with the 'Set advanced options' button to set the conversion you like for all images (e.g. resizing - this should always be done with 'Use Resample function' for high quality; otherwise, it makes no sense at all as it results in bigger files).
Finally, the output format should be set to TIFF CCITT Fax 4 compression. This automatically changes the images to monochrome, as the format only allows black and white pixels.