xv

The xv program displays images in the GIF, JPEG, TIFF, PBM, PGM, PPM, X11 bitmap, Utah Raster Toolkit RLE, PDS/VICAR, Sun Rasterfile, BMP, PCX, IRIS RGB, XPM, Targa, XWD, possibly PostScript, and PM formats on workstations and terminals running the X Window System, Version 11.

xv can write images in a variety of formats, with many of the modifications you may have made to the picture saved as well. You can use xv to do format conversion.

To start xv, enter the command: xv
To open a file in xv: xv filename

An online manual is available here.