How to remove sweat stains from pictures ?

There is another way, depending on the nature of the material and its texture, but you could simply paint it in with a soft brush.  Paint in any seams, add the texture, job done.   The fix below was done entirely with a soft brush sampling from around the problem area, and painting in the shadows.  The seams were stroked paths.

In this instance there was not enough texture to worry about, but you could add the texture to a 50% grey layer set to Overlay, and use a Displacement map to distort the texture to match the folds of the material.   Hand painting is a useful tool, and not hard to do.

When I remove spots I will use different methods which always include using the Patch Tool and/or the Clone Tool. What level would you describe yourself in editing skills? Frequency Separation is a method which I absolutely love because it allows you to change color without affecting texture and vice versa. I am including a link to a video that may help with that.

The Amazing Power of Frequency Separation Retouching in Photoshop - YouTube

Aaron Nace with Phlearn is helpful. There are a few things you can change as well within this technique. If you want to learn more I will try to get a video prepared for you showing the options I choose.

You are also able to create new layers that are transparent and paint the color of the shirt that is not wet over the stained spots. Select from different areas. Then play with blend modes and blend if to make it look and feel natural. Here is an example:

In this image - you can see the before had many many wrinkles. I used the clone tool, the patch tool, and a soft brush on a couple layers selecting colors from the dress, to paint. I also played with the blend modes and blend if options. Another thing I do it play with light whether with dodge and burning or simply painting with light. If you want some more tutorials or explanation let me know. Hope this helps some.

Show us your photos. From simple color adjustments constrained by selections to more elaborate clone and paint work this could be anything any any number of combinations, but nobody can tell anything from vague posts with nothing to look at.

Mylenium

I moved this topic from Photography​ to the Photoshop General Discussion​

Not sure if you still have this question after three months, but I recommend either the Clone tool or Patch tool. Using the Patch Tool and Clone Stamp Tool - Adobe Photoshop CS6 Tutorial - YouTube