本專案可以檢視硬碟中的圖片，完成品如下

說明

左邊的縮圖，為ListWidget中加入QPushButton，QPushButton中置入Icon. 右邊顯示大圖的部份為QLable。讀取硬碟圖片時, 時間非常長, 所以將讀取圖片的任務置於QThread中，待讀取完一張，就放入ListWidget一次。所以就會有讀一張顯示一張的效果了。

import os
import sys
from PyQt5.QtCore import QThread, pyqtSignal, QSize
from PyQt5.QtGui import QPixmap, QIcon
from PyQt5.QtWidgets import QMainWindow, QApplication, QPushButton, QListWidgetItem, QFileDialog
from ui.ui_mainwindow import Ui_MainWindow
class PicThread(QThread):
    callback=pyqtSignal(object)
    def __init__(self, path, parent=None):
        super().__init__(parent)
        self.path=path
        self.runFlag=True
    def run(self):
        ls=os.listdir(self.path)
        files=[]
        for l in ls:
            ll=l.lower()
            if ll.endswith('.jpg') or ll.endswith('.png'):
                files.append(f'{self.path}/{l}')
        index=0
        while index<len(files) and self.runFlag:
            pix = QPixmap(files[index])
            pix = pix.scaled(400, 300)
            pix.tag=files[index]
            self.callback.emit(pix)
            index+=1
            QThread.msleep(10)  # 要停一下, 否則UI無法立即顯示
class MainWindow(QMainWindow, Ui_MainWindow):
    def __init__(self, parent=None):
        super().__init__(parent)
        self.setupUi(self)
        self.path=os.path.abspath(os.path.curdir)
        self.lblPath.setText(self.path)
        self.btnPath.clicked.connect(self.btnPath_click)
        self.thread=PicThread(self.path)
        self.thread.callback.connect(self.refresh)
        self.thread.start()
    def btnPath_click(self):
        self.path = QFileDialog.getExistingDirectory()
        if self.path != '':
            self.lblPath.setText(self.path)
            self.lsIcon.clear()
            self.thread = PicThread(self.path)
            self.thread.callback.connect(self.refresh)
            self.thread.start()
    def btn_click(self):
        btn=self.sender()
        pix=QPixmap(btn.tag)
        w = pix.width()
        h = pix.height()
        r = w / h
        sw = self.lblImg.width()
        sh = self.lblImg.height()
        sr = sw / sh
        if r > sr:
            pix = pix.scaled(sw, sw / r)
        else:
            pix = pix.scaled(sh * r, sh)
        self.lblImg.setPixmap(pix)
    def refresh(self, pix):
        btn=QPushButton()
        btn.setIcon(QIcon(pix))
        btn.setIconSize(QSize(400,300))
        btn.tag=pix.tag
        btn.clicked.connect(self.btn_click)
        item=QListWidgetItem()
        item.setSizeHint(QSize(400,300))
        self.lsIcon.addItem(item)
        self.lsIcon.setItemWidget(item, btn)
if __name__=='__main__':
    app=QApplication(sys.argv)
    frame=MainWindow()
    frame.showMaximized()
    app.exec()