How to scan big pictures
First scan your image in as many overlapping scans (all at the same scale) as necessary to scan the whole picture.
Then patch together your scans in Photoshop or other photo manipulation software (the Gimp is free software you can download) to make the whole picture (using transparency on the layers to check that each piece overlaps correctly). You may need to slightly rotate some of the patches to ensure they overlap correctly.
Once it all fits correctly, change the transparency on all the layers back to 100% and use the eraser tool to gently rub away the hard edges on each layer.
Finally use curves, levels, brightness and contrast, hue and saturation or whatever you want to improve the appearance of your picture.
Flatten the picture, and save for web to post for all to see.
Or say ‘Sod it, nothing is worth that palaver’ and get out your digital camera.