I'm not 100% sure what you want. Loosely speaking, your images look like there are hills and valleys, with light coming from the left. And I think you're looking for the "valleys" in that landscape.
A mathematical model for this would be: There's a "height" (or "depth") for every pixel, and the image you have is the gradient (in X-direction) of that height field. So we want to find a height field, that, when convolved with a gradient filter kernel, reproduces your source image.

(this plot shows one line of the input and result images below)
This problem is called "Deconvolution", and Mathematica has a built-in function for it: ImageDeconvolve
.
Let's try this with one of your images:
urls = {"https://i.stack.imgur.com/LiQsY.jpg",
"https://i.stack.imgur.com/CRkrS.jpg",
"https://i.stack.imgur.com/cXTRq.jpg",
"https://i.stack.imgur.com/KUcKE.jpg",
"https://i.stack.imgur.com/OO36H.jpg"};
img = ImageTake[ColorConvert[Import[urls[[1]]], "Grayscale"], 500, 500];
I'm using a derivative of Gaussian filter kernel for the deconvolution, and (determined by trial and error) the "Wiener" method:
deconv = ImageAdjust@
ImageDeconvolve[img, GaussianMatrix[3, {0, 1}], Method -> "Wiener"]

This doesn't look too impressive, but if you binarize it (default threshold, no manual tinkering needed):
HighlightImage[img, Binarize[deconv]]

You see that we get a pretty good estimate for the "hills". Getting the "valleys" is just as easy:
HighlightImage[img,
Erosion[ColorNegate[Binarize[deconv]], DiskMatrix[5]]]

Here's the result for the full images:
Monitor[
Do[
img = Image[ColorConvert[Import[urls[[i]]], "Grayscale"],
ImageSize -> 1280];
deconv =
ImageAdjust@
ImageDeconvolve[img, GaussianMatrix[3, {0, 1}],
Method -> "Wiener"];
Export["so_Valleys" <> ToString[i] <> ".jpg",
HighlightImage[img,
Erosion[ColorNegate[Binarize[deconv]], DiskMatrix[5]]]]
, {i, Length[urls]}], i];



In the next two images, some of the "valleys" are brighter than the others, so the "brightness is gradient of height" model results in "slanted" areas: That's why these valleys are slightly larger on one side

