Didger 4
This is very simple to do. Import either the aerial photo or the DRG file using File | Import. When that file is imported, go back to File | Import and import the other file. Alternative to File | Import, you could also use Image | Download TerraServer Images multiple successive times to download different types of imagery into the same project.

Didger 3
To overlay DRGs and photos in Didger 3, this must be done in a vector project and all images (DRGs and photos) need to be georeferenced in the exact same projection. To do this:

1. First, I suggest you import one of the DRGs into a raster project in Didger (File | Import Bitmap | Into Raster Project), making sure to specify Embedded GeoTIFF Parameters as the spatial reference source during import. Then go to Image | Calibrate Image and note what projection and datum the DRGs are calibrated in. Make sure to click on the Settings button and note the settings for the projection as well.

2. Once you know the projection/settings/datum for the DRG files, you need to compare it to the aerial photo(s). Import the one of the photos into a raster project in Didger (File | New and then File | Import Bitmap | Into Raster Project). If the photo is not already georeferenced, go to Image | Calibrate Image and calibrate the image. If the photo is already georeferenced, go to Image | Calibrate Image and note the projection information.

3. If the photo is not in the same projection as the DRGs you will need to go to Image | Warp Image to warp the image, and then Image | Convert Image Projection and convert the image to the exact same projection/settings/datum that the DRG files are in. This may take a while depending on how large the photo file is.

4. Once the projection for the photo image matches the DRG, go to File | Export and export it out as a TIF file, saving the spatial reference information in GeoTIFF Format.

5. Repeat steps 2-4 for all aerial photo files you have.

6. Now you have the DRGs and the photos calibrated in the same projection. Import the DRG into a vector project in Didger (File | New and then File | Import Bitmap | Into Vector Project) and then go back to File | Import Bitmap | Into Vector Project and import one of the photos. You can repeat this and import as many images files as you like into the vector project.