You can replace a picture that is currently in the gallery with an improved picture such as a significantly larger version, a version without text, or a version without defects like creases, gaps, or incomplete character images.
Bigger is not always better when it comes to scans. Please pay attention to your print source. Submitting an extra large scan from a low DPI image source does not make it better quality. If the source is low quality, the scan is still low quality.
When uploading an improved version of a picture, use the "Improve" button next to the picture in the gallery to submit the new version. If you have scanned a better version of the same image yourself, you should upload it as a new submission. The "Improve" button is currently still under development, so please submit an improvement as a new scan.
As pictures are looked upon as image sources for anime wallpapers, the largest, most complete version with the least amount of defects on the usable character image is the ideal. If two pictures have no text, then the version with more complete characters (less cropped) would be given priority.
Even if a picture has no text or defect, if it is significantly smaller than the version in the gallery it will not be accepted. A Re-CG'ed image does not fall under this category.