Are you using one form and displaying different images depending on a condition or are there multiple forms?
As Crystal suggested if you are viewing images png or jpg are best.
With a form open in Design view, you can add images (pdf or jpg) to the Image gallery that can then be used on any form, report, or control.
Browse to an image and select it. The image will be inserted on the form and added to Shared Images. You can delete the image from the form by pressing escape or deleting the control.
To add the images to forms open each form in design and choose the required image from the insert Image Gallery.
The images are stored in the hidden table MSysResources. The name is the name used by the "Picture" property of the control where the "Picture Type" is "Shared".
If you will be displaying different images depending on a condition then you have 2 options.
1 Use MSysResources and make the Name of the image relate to the condition. You can rename by right-clicking the image in the gallery. When the condition changes you change the "Picture" property to the Name from MsysResources and the image will change.
2 Having added your images to MSysresources make a copy of the table, adjust the image names as required and use the new table as a source for the images on your forms. You can then remove the images from the gallery by right-clicking and deleting them.
MSysresources is automatically created by Access for Accdb 2010 and above. It initially has 1 entry which is the default theme. Usually Office Theme.