Change your text field into an unstored Calculation field (calculation result is text) and use this calculation:
Let ( [
i = GetValue ( GetAsText ( your_image_container_field ) ; 2 ) ;
p = Substitute ( i ; "/" ; "¶" )
GetValue ( p ; ValueCount ( p ) )
The calculation will return the name of the file and its extension, eg. image_035.jpg
Thanks .... again!