Where's your button to open the form?? Don't put the button in the form header - put it in the detail section so that it appears on every row. That section should also contain the textbox with the file path. The button click event should test for txtFilePath being Null or an Empty string and if not, open the form with the image control on it and set the image property to be the path for the chosen record. I know that if you do this on a report, it will know which record the button is on, so I don't see why it won't work for a form.
Before you go too deep, make the button click event to Msgbox Me.txtSomeBoxThatIsn'tBlank. You should get the correct value in the message for that row. Once that's working, delete that and write the code to open the form instead. I would probably pass the path as the OpenArgs parameter of the DoCmd.OpenForm method although there are other ways. AFAIC they're not as simple. You can then set the picture path property of the image control to whatever you pass to the OpenForm method.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.