The code

# Declaration of the folder containing the files to rename
$rootFolder = "C:\_DEV\inzertnistranky\images"

# The function for removal of accented and non-standard characters within the string (Regular Expressions used - make sure they are escaped properly)
function Remove-AccentedCharactersAndSpaces {
    $normalizedString = $inputString.Normalize('FormD')
    $asciiOnly = $normalizedString -creplace "[^\x20-\x7E]", "" #Removal of each accented character in the string.
    $noSpaces = $asciiOnly -replace '\s', '-'   #The replacement of space character by minus.
    $nounderscores = $noSpaces -replace '_', '-'   #The replacement of underscore symbol to minus.
    $noequales = $nounderscores -replace '\=', ''      #Removal of symbol equals to.
    $noleftbracket = $noequales -replace '\(', ''      #Removal of left bracket.
    $norightbracket = $noleftbracket -replace '\)', ''      #Removal by right bracket.
    $nodoubleminus = $norightbracket -replace '\-\-', '\-'      #The removal of the multiminuses and replaced by single minus.
    return $nodoubleminus.ToLower()   #The function returns the name of the file in lowercase string.

# Funkce pro rekurzivní přejmenovávání souborů
function Rename-FilesRecursively {
    param([string]$folderPath)  #The parameter of the function - the path to the files
    # Get each files in the folder as the object
    $files = Get-ChildItem -Path $folderPath -File  #The files array
    # To rename each file
    foreach ($file in $files) {     #File objects in files array
        $newFileName = Remove-AccentedCharactersAndSpaces -inputString $file.BaseName   #Call the function for accents and spaces removal
        if ($newFileName -ne $file.BaseName) {  #If new filename does not comply to the previous filename, generate the variables for renaming
            $extension = $file.Extension    #extension
            $newBaseName = $newFileName + $extension    #new filename with extension
            $newFilePath = Join-Path -Path $file.Directory.FullName -ChildPath $newBaseName #nová cesta k přejmenovanému souboru
            Rename-Item -Path $file.FullName -NewName $newFilePath -Force   #Renaming of the file  to new name after accents and spaces were replaced
    # Recursive call for each subfolder
    $subfolders = Get-ChildItem -Path $folderPath -Directory    #The object for subfolders
    foreach ($subfolder in $subfolders) {  
        Rename-FilesRecursively -folderPath $subfolder.FullName #The call of the recursive function for renaming of each file in the subfolder

# The calling for the function initiating the rename of the files
Rename-FilesRecursively -folderPath $rootFolder