Image linked to table

I have a dbase that I developed a few years ago using Access 2003. I have a
form that brings together a variety of information about a student based on a
query using several tables. One of the tables is the tblImages. The link is
the student ID. I have a folder in the same directory that contains images
of the students. In the database, I have a table called tblImages. The
fields in the table are Student ID and a link to the image in the external
directory. I right click on the field and select import object then click
the link box and locate the image. I used to be able to link a bmp image
this way with no problem. Now I can only get it to work with a pdf image
which usually appears pixalated. How can I use this method with a jpg, png
or gif?
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jj Trying to learn