Please raise issue if you know better or alternative ways to convert and process images using Java

Convert jpeg, png, bmp, wbmp, and gif into jpg using Java (only )

io.szjug.ImageConverter7 is enhancement over examples listed below.
It uses only JDK (jpeg, png, bmp, wbmp, and gif),
tif is supported since Java 9 incl Java 11.

png to jpg conversion is relatively easy http://www.mkyong.com/java/convert-png-to-jpeg-image-file-in-java/

https://www.roseindia.net/java/example/java/swing/ConvertImageToFormats.shtml

https://www.codejava.net/java-se/graphics/convert-image-formats

Some theory:

• https://en.wikipedia.org/wiki/JPEG
  Magic number ff d8 ff
• https://en.wikipedia.org/wiki/Portable_Network_Graphics
  Magic number 89 50 4e 47 0d 0a 1a 0a
• https://en.wikipedia.org/wiki/TIFF

Alternatives to Java 9:

• https://github.com/jai-imageio/jai-imageio-core
• https://github.com/geosolutions-it/imageio-ext (wiki has much information)
• https://github.com/haraldk/TwelveMonkeys (see https://haraldk.github.io/TwelveMonkeys/)