Mosaic

The MosaicPro tool in IMAGINE Advantage and LPS creates and 

fine-tunes large mosaics that may include thousands of images. Offering dynamic update visualization, MosaicPro efficiently facilitates the seam-editing process.

Step 1,

Open ERDAS -- Raster -- Mosaic -- MosaicPro

Step 2,

Select "Display add Images Dialog" button to add images to be Mosaic

Step 3,

Select the Image you need..

Step 4,

To view the image press "Display Raster Images" button

Step 5,

Add another image as previous

Step 6,

Then go to Process -- Run Mosaic 

Step 7,

Select file type as you need the output 

Step 8,

Step 9,

If you need you can use this outputs in ArcGIS also..