Modifications done in order to make the picture more "appealing" to the human eye, will most likely lead to it's disqualification. The key here is that these kind of modifications have to add something to the base picture and not detract from it.

Cropping is usually okay, as are adjustments to color balance, contrast, and brightness; however, any changes that alter the content of the photo may render it ineligible for entry into certain contests. For instance: cloning (copying part of the image and pasting it somewhere else), adding objects, removing objects, changing backgrounds, etc., may all result in a photo being rejected by contest administrators. In addition, some competitions require that photos be submitted as RAW files rather than JPEGs in order to ensure their authenticity, so check the rules carefully before submitting an entry.
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