When using GUIDE, the process for this is typically as follows.
1. In a callback, use uigetfile to grab file name and then load the data.
2. Add the data to the handles structure.
>> myData = imread('...')
>> handles.myData = myData;
3. Update the GUI's version of the handles structure using guidata.
>> guidata(hObject, handles)
4. Then, in subsequent callbacks, the data can be accessed in the handles structure:
>> image(handles.myData)
More info on guidata can be found here
There are other ways as well, but this approach is very common when using GUIDE. Hope this helps!