Any image file to be worked in ERDAS is to be converted into Image(*.img) format. Observe the video to convert any type of file to .img format. Here in the below example I've used JPEG image, similarly ERDAS supports many formats to be used.
Go to ERDAS Imagine
Click on Import/Export Module
Specify the input format from the drop down box
Browse the input file
Give the destination folder for Output file and click on ok