We added functionality to generate thumbnails for all images in folder/folders (including sub-folders or not). Please see the usage below:

To create a thumbnail, you should install the component and provide the valid license key in Components -> ARI Thumbnail -> License section. After that you will be able to use plugin (mambot) with the folowing syntax:
{arithumb
src="big_img"
srcSmall="small_img"
srcDir="src_dir"
includeSubDirs="true|false"
title="title"
thumbWidth="width"
thumbHeight="height"
group="group"
hideOnContentClick="true|false"
zoomSpeedIn="speed"
zoomSpeedOut="speed"
overlayShow="true|false"
overlayOpacity="capacity"}
Attributes:
src - Path to the image. (Mandatory)
srcSmall - Path to the thumbnail. If not indicated then thumbNail will be automatically generated using thumbWidth and thumbHeight attributes. If one of these attributes is skipped then the thumbnail will be generated using the indicated attribute and initial picture proportion. Thumbnail autogeneration feature needs GD, ImageMagick or magickwand library installed. (Optional)
srcDir - Indicating folder (or folders separated by ";" lets to display thumbnails for all images in directory/directories. This can be done including subfolders if includeSubDirs is true (true by default).
includeSubDirs - see srcDir description.
title - Image caption. (Optional)
thumbWidth - Width of thumbnail to be generated from the initial image. (Optional)
thumbHeight - Height of thumbnail to be generated from the initial image. (Optional)
group - all images with the same "group" attribute will be displayed as image gallery. (Optional)
hideOnContentClick - Hides image view when clicked on zoomed item. (Optional, false by default)
zoomSpeedIn - Speed in milliseconds of the zooming-in animation. (Optional, no animation if 0)
zoomSpeedOut - Speed in milliseconds of the zooming-out animation. (Optional, no animation if 0)
overlayShow - If true, shows the overlay. (Optional, false by default)
overlayOpacity - Opacity of overlay. (Optional, from 0 to 1)