There are are lot of "legal" things you can do to enhance an image, including crop/rotate, tone adjustments (Levels/Curves), color adjustments, grayscale/duotone, sharpening, blurring, noise reduction, etc. The major limitation is that these must be applied globally to the entire image, not to selected portions.
There are many EFFECTS (usually filters) which are NOT legal, and techniques such as layering and compositing images is usually prohibited. Some of these may be allowed for specific challenges, so always read the "extra rules" with each challenge as well as reviewing the general rules. |