You can use MicroDEM to add WGS84 or NAD83 datum info to GeoTiff files (and to re-project files in any of the datums acceptable to MicroDEM into either WGS84 or NAD83).

As an example using an image saved with USAPhotoMaps, you'd use the following steps:

1. Use USAPhotoMaps to create a JPEG image and world file.

2. Start up MicroDEM 8, and in Options set the default projection to the one you want to save your file in. (It must be either NAD83 or WGS84; MicroDEM can read many other projections, but can only save GeoTIFF files in those two.)

3. Choose "Open Image" from the file menu. In the "Files Of Type" drop-down menu, choose "BMP/JPEG/PNG, world file". Navigate to the folder that contains the JPEG and world file created in step 1, and open it. Specify its original UTM zone and datum (NAD83 for USAPhotoMaps images) in the window that pops up.

4. Click on the icon of a magnifying glass with a "1" in the center of it to zoom the picture to full 1:1 scale. (Note: if the image is very big, you may have to go into options to change the maximum allowable map size.). If you want to enhance the image, right-click on it, choose "Contrast" from the drop-down menu, and pick the option you want.

5. From the File menu, select "Save Map As Image => GeoTIFF, Screen Scale".

6. MicroDEM will now save the file as a GeoTIFF with the projection and datum data embedded.

You can also use this technique to convert files already georeference in one projection, like NAD27 files (e.g. most USGS DRG topo files), into WGS84 or NAD83 projection. Just open the file, expand it to 1:1 scale, then save it as a GeoTIFF. 

Also, see Image Reprojection Utility - DGWarp