Proceed to Apps > PicManager: Image Uploader and check the matching criterion selected for the uploading process. Make sure your image names correspond to the SKUs, product titles or the titles of the product variants:
Check the match type: if the full match is selected, the image title should fully correspond to the SKU or the product title on your store:
Finally, during the match, make sure you have finalized the upload by clicking the Finish Upload button: