To assign the same image(s) to several products at once, you can try using the Manual match mode. To proceed, navigate to Apps > PicManager: Image Uploader > Upload and select the Manual assign option:
As the next step, upload your image and click Start Uploading.
Choose to upload images to selected products:
Once done, you will get a pop-up where you can select the products to upload the images to:
You can select one or several products by clicking the checkboxes in the pop up and click Select. As the next step, please, click Continue:
And finalize the upload: