If you want to convert and compress JPEG files on Linux Ubuntu without losing the quality, you may want to try wxPackJPG application. This GUI JPEG Compression tool will help you to reduce the file size of the picture without losing or reducing the quality.
wxPackJPG is a multiplatform application which available for Windows, Linux and Mac. wxPackJPG builds upon packJPGlib to losslessly compress JPEG image files by about 20%. It can be used to archive JPEG images. It features a nice GUI, multithreading, multiplatform capability and offers a portable version which does not store anything in the Windows registry or in .ini files.
Want to give wxPackJPG a test? Download it for free from this link.