To allow the user to select a file (including path) so that you can then reference that file to display it in the image control, use the Open/Save Dialog. This allws the uses of the familiar Windows file access dialog boxes without a lot of coding.
You can use the Common Dialog Control (in which there is a bounty of on-line help for) but I have found the Common Dialog Controls to be unreliable (especially when multiple users, PC and environments come into play). If still interested, make sure you have Common Dialog Boxes referenced and checkout MSDN for help on using it.
Yep, one of several problems with using the common dialog control.
If you are still interested, there is code in the other thread I listed that utilizes the built in dialog boxes in Access, avoiding the need (and potential for problems) with using the outside OCX controls.
Ken has it setup so that it's as simple as using a function to get your filename back.