QButtonGroup

import sys
from PyQt5.QtWidgets import QApplication, QWidget, QMainWindow, QLabel
from PyQt5.QtGui import QPixmap
from table_design import Ui_MainWindow as Ui_Table
from element_design import Ui_MainWindow as Ui_Element
 
SCREEN_SIZE = [250, 200]
last_pressed_element = None
 
 
class TableM(QMainWindow, Ui_Table):
    def __init__(self):
        super().__init__()
        self.setupUi(self)
        self.all_elements.buttonClicked.connect(self.open_second_form)  # кнопки, объединённые в QButtonGroup
 
    def open_second_form(self):
        global last_pressed_element
        last_pressed_element = self.sender().text()
        self.second_form = OpeningForm()
        self.second_form.show()
 
 
class OpeningForm(QMainWindow, Ui_Element):
    def __init__(self):
        super().__init__()
        self.setupUi(self)
 
        self.simple_element = QPixmap(f'Простые вещества/{last_pressed_element}.jpg')
        self.picture.setPixmap(self.simple_element)
 
 
if __name__ == '__main__':
    app = QApplication(sys.argv)
    ex = TableM()
    ex.show()
    sys.exit(app.exec())